CA1298638C - Optical signal processing device - Google Patents
Optical signal processing deviceInfo
- Publication number
- CA1298638C CA1298638C CA000554367A CA554367A CA1298638C CA 1298638 C CA1298638 C CA 1298638C CA 000554367 A CA000554367 A CA 000554367A CA 554367 A CA554367 A CA 554367A CA 1298638 C CA1298638 C CA 1298638C
- Authority
- CA
- Canada
- Prior art keywords
- amplifier
- optical signal
- optical
- intensity
- input
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F3/00—Optical logic elements; Optical bistable devices
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/50—Amplifier structures not provided for in groups H01S5/02 - H01S5/30
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
- H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
Abstract
- ABSTRACT -OPTICAL SIGNAL PROCESSING DEVICE
By using a DFB semiconductor laser amplifier 1 in reflection, it is possible to obtain a device with an optical limiting characteristic. The output signal intensity of such an amplifier 1 is substantially independent of the input signal intensity where the input signal is detuned from an output peak on the short wavelength side of a stop band of the amplifier 1, and has an intensity above a threshold value.
The amplifier 1 finds particular application for instance as anoptical limiter or noise filter , in optical logic and communications systems;
By using a DFB semiconductor laser amplifier 1 in reflection, it is possible to obtain a device with an optical limiting characteristic. The output signal intensity of such an amplifier 1 is substantially independent of the input signal intensity where the input signal is detuned from an output peak on the short wavelength side of a stop band of the amplifier 1, and has an intensity above a threshold value.
The amplifier 1 finds particular application for instance as anoptical limiter or noise filter , in optical logic and communications systems;
Description
F~OM ~ S~ ' 37 . 12. 1~; 15: 41 ` ; ~29~3638 rs !
,,. 1 --OPTICAL SIGNAL PROCESSING ~EYICE
The present invent;on relates to an optical s;gnal process~ng devlce. It finds particular application in optical log;c and commun;cations systems, for instance as an opt;cal signal limiter, or as a ~oise f~l~er, .. . . , . . .. ,.. ... .... . . , ., .. , i , ...... . .
It is sometimes useful in optical logic or communications systems that the lntensity of an optical signal is limited to a ma~lmum value which ~s at least substantial~y constant. Such a limited signal could .for Instance ~e used as a b~as s~gnal, to ensure reproducable peak laser power in pulse~ laser systems, to con~rol pulse shapes, as a d1gital input signal wh~ch avoid~ saturat~ng a photodetector, or to equal;se signal ampl~tudes in a multiplex system. further, a deYlce whlch ll~ts opt~cal signal intenslt~es can operate as a no~se fflter s~nce modulation sup~ri~nposed on an optical s~gnal can be cut out.
It Is known to use semlconduct~r optlc~l deYices ~n optical log~c and s~gnal processing; They are advantageous ~n that they can be deslgned to oparate at t~
order of low power levels, for Instance at about ~ 10 ~W
which might be aYa~ le ~n optical log~c and stgnal process~ng. They phys~cally take up l~ttle space, operate at wavelengths compatible with those co~mon in opt~cal logic and comm~nlcatiors, and potentially can be monolithically lntegrated with other opt;cal components.
A factor in the choice of materials for optical devices is the fact that s;lica opt;cal fibres, which are the basis of present optical communications systems, have loss min;ma at 0.9 ~m, 1.3 ~m and 1.55 ~m approximately.
Accordingly there is an especial need for deYices ;~lich FI~OM ~1 33~ ` R7 . ~ ~ . 15 I S: 4:~
; ~LZ 9E36 3 8 show favourable character~stlcs when operated us~ng opt1cal rad~at~on ~n the wavetength range from 0.8 to 1.65 um, and espec~ally in the ranges from 0;8 to 1.0 ~m and from 1.3 ~m to 1.65 ~. (These wavelengths, l~k~ al1 the waveleng~hs here~n except where the con~ex~ ~ndfcates otherw~se, are in vacuo waveleng~hs); Mater~als whlch have been found sultable for the manufacture of devices which show such favourable character~stlcs comprlse the III-V sem~conductor ma~erials, ~nclud~ng gall~um arsenide, ind~um gall~um arsen~de~ gall~um allum~n~um arsenide, ~ndium phosph~de and the quaternary mater1als, ~nd~um gall~um arsen~de phosph~des ~Inx Ga1 x Asy Pl y)~
Wlth regard to the quaternary mater~als, by sultable cho~ces of x and y it ~s poss~ble to latt~ce-match reg~ons of d~fferent ones of these mater~als to nelghbour~ng III-Y
materlals ln a device while belng able to select the assoclated band gap equivalent wavelength;
A known dev~ce for use in con~unctlon w~th a laser to shape pulses ~s a non l~near Fabry-Perot tFP) etalon. A
FP etalon ~s a s~mple optlcal cav~ty wh~ch can be fabr~cated from semlconductor materlals, hav~ng planar reflect~ng endfaces of about ~0~ reflect~v~typ and (optlonally~ wavegu~d1ng propert~es. L~m~tlng action ~s achieved by tun~ng the peak output of the etalon at low s~gnal input to the source laser frequency: as ~he input ~ntensity r~ses~ the etalon detunes, keep~ng the transm~tted power approx~mately constant;
Alternat~vely, passing a s~gnal through a non-linear med~um can be used ~o llm~t the slgnal. It ls known to .
use a dev;ce whereln the far field of the output signal spreads at ~ncreaslng input powers. By us~ng an aperture stopp the ~ncreasing ~nput powers can be at least approximately balanced out.
' F R ~ M e~
8~38 3;
Another dev~ce suggested but not demonstrated for use as an op~cal s~gnal ll~ter is a mult~ple quantum well guided wave structure, also fabr~cated from sem~conductor materials; Such a dev~ce is d~scussed ~n ~Nonl~near Gu~ded Wave Applicat10nsW, Opt~cal Engineerlng 24 (4) 1985, by CT Seaton, Xu Mai, GI Stegem3n and H G W~nful.
Opt~cal l~mitfng action ~s obtalned becawse either one or both of the bound~ng medla are character~zed by a s~lf-defocusing nonlinearlty.
HoweYer9 known devices tend to suffer fr~m one or more disadvantages. For instance, th~y might orly operate at an ~nput s~gnal intenslty hlgher than those su~tab7e for optical loglc or communicàtions systems; Another disadvantage, for lnstance of etalons, can be that the output slgnal is mult~stable and ~ends to show osc~llat~on as a consequence. In the case of a mult~ple quantum well devlcq~ ~t ls l~kely that eYen ~f demonstrated, the dev~ces would be difficult to manufacture, hav~ng for ~nstance low layer th~ckness tolerances;
A sem~conductor laser ls known for use as a source of opt~cal rad1at10n~ or ~s an ampl1f~er; A laser ~s commonly based on a wafer grown from the III-Y
semlcon~uctor materlals ~ent~oned above. The layers of the wafer are select~vely doped to provlde a p-n ~unct~on, in the vlc~n~ty of which lies an active region. Photons can be generated ~n the actiYe region by rad~atiYe recomb~nat~on of electron-hole (carrier) pairs under a dr~ving current appl~ed across the ~unct~on. ~he d~strlbution of refractive index in the laser is such as to prov~de a waYeguiding region along which the photons generated are gu~ded. Feedback is provided to the waveguiding region, for instance by reflect~ve end facets of the laser (a Fabry-Perot laser). In alternative versions, ~eedback may be provided in a "d~stributed"
FROM ell 3a0 ~:10 '~7. 12. 15 1~i:4'3 ~L2~ i3~3 fash~on )a DFB laser), for ~nstance by means of corrugat~ons ~n an ~nterfaçe which l~es near the ac~iYe reg~on, or by a gratin3 external to the laser act~ve/waveguidlng reg~ons.
If optical radiat~on is input to the active reg~on o~
a laser and a driv~ng ~urrent applied, ampllf~cation o~
the rad~at~on occurs even when the driviny current ~s below the ~hreshold current necessary for laslng actlon to occur. The relat~onship be~ween ~nput and output rad~at~on intens~$y ls non-linear and can show blstab~l1ty, the output ~ntensity swltching rapidly to a new value as the input intensity reaches a relevant switching leYel.
A feature of a DFB laser ~s that the input slgnal wlll be reflected to an extent dependent on its wavelength;
Any passive corrugated wavegu~de will act as a band reflect~on f~lter, for optlcal radiation of wavelength ~ ;~
free space, whose per~od J~L ~s close to ~he back ~catter1ng ~rogg condl~t~on ~L = p~ where p ~s an integer and n is the 2n effect~ve rePractive Index of the material of the wavegulde. This feature cre~tes a series of stop bands with~n wh~ch optical rad~atlon is substantially reflected rather than transmitted. In a DFB active device, the reflected spectral response ls more compl~cated,showlng a strong peak to e~ther side of each stop band, caused by the device ga~n. (In ~he presen~ specification, references to a stop band w~ll be to the stop band relevant at very low input signal intensities.) The optical radiation can be input to d first end of the act~ve region of a laser ampl~fier and the output taken from a second end of the açtive reg~on, ~e the laser amplifier can be operated ~n transmission. Alternatively, . '' , `
.
`
FRIIM 1~1 3S1~ ~SIg J ~7, 1~ 5 5~
~2~ i3 the output can be taken from the flrs~ end also, ie the laser ampl~fier can be operated in reflec~on;
The relatlonshlp between input and output rad~ation intensity is compl1cated. For Ins~ance, the input radiat~on undergo~ng amplif~eat~on reduces the free carr~er concentration and hence ~he ga~n of the laser;
The refractive Index var~es w~th galn, and the degree of ampllffca~lon of the input rad~at~on ~s dependent on the . .. . , .. ~, ... . .. , .. ~ . .. ....... ..
Pelatlonship between the ~nput ~aYelength and the refract~ve ~ndex. Additionally, temperature w1tl affect the refractl~e inde~ and both the laser drive conditions and the ~nput rad~ation affect the temperature. Hence, overall, the interact~on of ga~n, refractive ~ndex, driv~ng current and input rad~at~on is d~fP~cult to spec~fy for a particular laser.
It has now been found that a DFB laser used as an amp1~f~er can show an optical limitlng character~stic;
" ~t i,s an obiect of the present invention to provide an optical signal process~ng dev~ce suitable for us~ as an optical limiter ~n opt1cal logic or com~unications systems.
Accordlng to the present invent~on there ~s prov~ded an optical signal processing devlce comprls1ng a sem~conductor DFB laser ampl~er operdted in reflect~on, means for coupl~ng an optical ~nput s~gnal to the amplifier, the signal being detuned from an output signal peak to the short wavelPnQth s~de of the wa~elength range relevant to an ampl~fier stop band, and means for applying a drlYing current to the amplifier, the lnput signal reaching at least part of the time an intens;~y greater than a threshold value above which the output signal intensity of the amplifier is substant;ally independent ~)f the input signal intens;ty.
Preferably where the ~nput signal is a binary digital signal, its ;ntens;ty for a "1" value is always greater than that threshold value.
F R~iM ~ 0 ~ ~31~ 1 13 ~
~986~3 .
,,. 1 --OPTICAL SIGNAL PROCESSING ~EYICE
The present invent;on relates to an optical s;gnal process~ng devlce. It finds particular application in optical log;c and commun;cations systems, for instance as an opt;cal signal limiter, or as a ~oise f~l~er, .. . . , . . .. ,.. ... .... . . , ., .. , i , ...... . .
It is sometimes useful in optical logic or communications systems that the lntensity of an optical signal is limited to a ma~lmum value which ~s at least substantial~y constant. Such a limited signal could .for Instance ~e used as a b~as s~gnal, to ensure reproducable peak laser power in pulse~ laser systems, to con~rol pulse shapes, as a d1gital input signal wh~ch avoid~ saturat~ng a photodetector, or to equal;se signal ampl~tudes in a multiplex system. further, a deYlce whlch ll~ts opt~cal signal intenslt~es can operate as a no~se fflter s~nce modulation sup~ri~nposed on an optical s~gnal can be cut out.
It Is known to use semlconduct~r optlc~l deYices ~n optical log~c and s~gnal processing; They are advantageous ~n that they can be deslgned to oparate at t~
order of low power levels, for Instance at about ~ 10 ~W
which might be aYa~ le ~n optical log~c and stgnal process~ng. They phys~cally take up l~ttle space, operate at wavelengths compatible with those co~mon in opt~cal logic and comm~nlcatiors, and potentially can be monolithically lntegrated with other opt;cal components.
A factor in the choice of materials for optical devices is the fact that s;lica opt;cal fibres, which are the basis of present optical communications systems, have loss min;ma at 0.9 ~m, 1.3 ~m and 1.55 ~m approximately.
Accordingly there is an especial need for deYices ;~lich FI~OM ~1 33~ ` R7 . ~ ~ . 15 I S: 4:~
; ~LZ 9E36 3 8 show favourable character~stlcs when operated us~ng opt1cal rad~at~on ~n the wavetength range from 0.8 to 1.65 um, and espec~ally in the ranges from 0;8 to 1.0 ~m and from 1.3 ~m to 1.65 ~. (These wavelengths, l~k~ al1 the waveleng~hs here~n except where the con~ex~ ~ndfcates otherw~se, are in vacuo waveleng~hs); Mater~als whlch have been found sultable for the manufacture of devices which show such favourable character~stlcs comprlse the III-V sem~conductor ma~erials, ~nclud~ng gall~um arsenide, ind~um gall~um arsen~de~ gall~um allum~n~um arsenide, ~ndium phosph~de and the quaternary mater1als, ~nd~um gall~um arsen~de phosph~des ~Inx Ga1 x Asy Pl y)~
Wlth regard to the quaternary mater~als, by sultable cho~ces of x and y it ~s poss~ble to latt~ce-match reg~ons of d~fferent ones of these mater~als to nelghbour~ng III-Y
materlals ln a device while belng able to select the assoclated band gap equivalent wavelength;
A known dev~ce for use in con~unctlon w~th a laser to shape pulses ~s a non l~near Fabry-Perot tFP) etalon. A
FP etalon ~s a s~mple optlcal cav~ty wh~ch can be fabr~cated from semlconductor materlals, hav~ng planar reflect~ng endfaces of about ~0~ reflect~v~typ and (optlonally~ wavegu~d1ng propert~es. L~m~tlng action ~s achieved by tun~ng the peak output of the etalon at low s~gnal input to the source laser frequency: as ~he input ~ntensity r~ses~ the etalon detunes, keep~ng the transm~tted power approx~mately constant;
Alternat~vely, passing a s~gnal through a non-linear med~um can be used ~o llm~t the slgnal. It ls known to .
use a dev;ce whereln the far field of the output signal spreads at ~ncreaslng input powers. By us~ng an aperture stopp the ~ncreasing ~nput powers can be at least approximately balanced out.
' F R ~ M e~
8~38 3;
Another dev~ce suggested but not demonstrated for use as an op~cal s~gnal ll~ter is a mult~ple quantum well guided wave structure, also fabr~cated from sem~conductor materials; Such a dev~ce is d~scussed ~n ~Nonl~near Gu~ded Wave Applicat10nsW, Opt~cal Engineerlng 24 (4) 1985, by CT Seaton, Xu Mai, GI Stegem3n and H G W~nful.
Opt~cal l~mitfng action ~s obtalned becawse either one or both of the bound~ng medla are character~zed by a s~lf-defocusing nonlinearlty.
HoweYer9 known devices tend to suffer fr~m one or more disadvantages. For instance, th~y might orly operate at an ~nput s~gnal intenslty hlgher than those su~tab7e for optical loglc or communicàtions systems; Another disadvantage, for lnstance of etalons, can be that the output slgnal is mult~stable and ~ends to show osc~llat~on as a consequence. In the case of a mult~ple quantum well devlcq~ ~t ls l~kely that eYen ~f demonstrated, the dev~ces would be difficult to manufacture, hav~ng for ~nstance low layer th~ckness tolerances;
A sem~conductor laser ls known for use as a source of opt~cal rad1at10n~ or ~s an ampl1f~er; A laser ~s commonly based on a wafer grown from the III-Y
semlcon~uctor materlals ~ent~oned above. The layers of the wafer are select~vely doped to provlde a p-n ~unct~on, in the vlc~n~ty of which lies an active region. Photons can be generated ~n the actiYe region by rad~atiYe recomb~nat~on of electron-hole (carrier) pairs under a dr~ving current appl~ed across the ~unct~on. ~he d~strlbution of refractive index in the laser is such as to prov~de a waYeguiding region along which the photons generated are gu~ded. Feedback is provided to the waveguiding region, for instance by reflect~ve end facets of the laser (a Fabry-Perot laser). In alternative versions, ~eedback may be provided in a "d~stributed"
FROM ell 3a0 ~:10 '~7. 12. 15 1~i:4'3 ~L2~ i3~3 fash~on )a DFB laser), for ~nstance by means of corrugat~ons ~n an ~nterfaçe which l~es near the ac~iYe reg~on, or by a gratin3 external to the laser act~ve/waveguidlng reg~ons.
If optical radiat~on is input to the active reg~on o~
a laser and a driv~ng ~urrent applied, ampllf~cation o~
the rad~at~on occurs even when the driviny current ~s below the ~hreshold current necessary for laslng actlon to occur. The relat~onship be~ween ~nput and output rad~at~on intens~$y ls non-linear and can show blstab~l1ty, the output ~ntensity swltching rapidly to a new value as the input intensity reaches a relevant switching leYel.
A feature of a DFB laser ~s that the input slgnal wlll be reflected to an extent dependent on its wavelength;
Any passive corrugated wavegu~de will act as a band reflect~on f~lter, for optlcal radiation of wavelength ~ ;~
free space, whose per~od J~L ~s close to ~he back ~catter1ng ~rogg condl~t~on ~L = p~ where p ~s an integer and n is the 2n effect~ve rePractive Index of the material of the wavegulde. This feature cre~tes a series of stop bands with~n wh~ch optical rad~atlon is substantially reflected rather than transmitted. In a DFB active device, the reflected spectral response ls more compl~cated,showlng a strong peak to e~ther side of each stop band, caused by the device ga~n. (In ~he presen~ specification, references to a stop band w~ll be to the stop band relevant at very low input signal intensities.) The optical radiation can be input to d first end of the act~ve region of a laser ampl~fier and the output taken from a second end of the açtive reg~on, ~e the laser amplifier can be operated ~n transmission. Alternatively, . '' , `
.
`
FRIIM 1~1 3S1~ ~SIg J ~7, 1~ 5 5~
~2~ i3 the output can be taken from the flrs~ end also, ie the laser ampl~fier can be operated in reflec~on;
The relatlonshlp between input and output rad~ation intensity is compl1cated. For Ins~ance, the input radiat~on undergo~ng amplif~eat~on reduces the free carr~er concentration and hence ~he ga~n of the laser;
The refractive Index var~es w~th galn, and the degree of ampllffca~lon of the input rad~at~on ~s dependent on the . .. . , .. ~, ... . .. , .. ~ . .. ....... ..
Pelatlonship between the ~nput ~aYelength and the refract~ve ~ndex. Additionally, temperature w1tl affect the refractl~e inde~ and both the laser drive conditions and the ~nput rad~ation affect the temperature. Hence, overall, the interact~on of ga~n, refractive ~ndex, driv~ng current and input rad~at~on is d~fP~cult to spec~fy for a particular laser.
It has now been found that a DFB laser used as an amp1~f~er can show an optical limitlng character~stic;
" ~t i,s an obiect of the present invention to provide an optical signal process~ng dev~ce suitable for us~ as an optical limiter ~n opt1cal logic or com~unications systems.
Accordlng to the present invent~on there ~s prov~ded an optical signal processing devlce comprls1ng a sem~conductor DFB laser ampl~er operdted in reflect~on, means for coupl~ng an optical ~nput s~gnal to the amplifier, the signal being detuned from an output signal peak to the short wavelPnQth s~de of the wa~elength range relevant to an ampl~fier stop band, and means for applying a drlYing current to the amplifier, the lnput signal reaching at least part of the time an intens;~y greater than a threshold value above which the output signal intensity of the amplifier is substant;ally independent ~)f the input signal intens;ty.
Preferably where the ~nput signal is a binary digital signal, its ;ntens;ty for a "1" value is always greater than that threshold value.
F R~iM ~ 0 ~ ~31~ 1 13 ~
~986~3 .
- 6 ;
Optic~l dev1ces accordlng to embodiments of the present Invent~on hava the advantage that the rel~t~onshlp between lnput and output slgnal in~ens~t~es can show an extenslve reglon wlth1n whlch the ou~put signal fntensity Is substant1ally ~ndependent of Input slgnal ~ntenslty;
Further, because the ampll~ler ls operated in reflect~on, the devlces can offer the ablllty to use samples of hfgher llnear absorptlon.
Because the dev~ces are aet~ve rather than passlve9 there 1s also cons~derable control ava~lable over the operd~ng parameters used.
The Input-output signal Intensity relatlonshlp of an ampllf~er dccordlng to an embodiment of the present lnventlon can be used to ll~lt nolse ln an optlcal signal slnce intensity variatlDns of the Input slgnal wil~ not be reproduced. Where the signal is a blnary dlgltal s~gnal~
no~se can be llm~ted not only on "1" values of the signal, where the slynal lntens~ty l~es above the threshold value, but also on "O" values. Th~s is because lf the Gauss~an spread of the slgnal lntens~ty, assoclat~d with nolse at the ~l" value, ~s reduced, then the dec~s~on level selected to d~fferentiate between ~ and a "Oll Yalue can be ra~sed, so reduclng the effect of noise on a .,Ou value at the decis~on leYel concerned;
An optlcal llmlter according to an embodlment of the present inventlon wlll now be descrlbed, by way of example only~ w~th reference to the accompanying flgures in wh~ch:
Flgure 1 shows a schematlc representation of the llmlter In use;
Flgure 2 shows in graph form the output signal of the limlter ~n response to var~ous Input signals of different wavelengths; and Figure 3 shows the response of the lim~er to a high frequency slnusoidal Input slgnal.
F l;~ O M ~11 3 æ 0 ~ a ,- . 12 . I 'i 1 ~J ', 4 ~2~86313 Referrlng to F~gure 1, the lim~ter compr~ses a DFB
ampllfler 1 used ln reflec~lon. An opt1cal ~nput signal ls prov~ded by a tunable laser dlode so~rce 2, ~n comblnatlon w~th an attenuator 7.
A beam spl~tter 3 mou~ted between the source 2 and the ampllf~er 1 deflects a port~on of the optlcal output of the source 2 to an lnput s~gnal monltor 6, ~nd a port~on of the output o~ the ampllfler 1 to an output stgnal mon~tor 5~ Interactlon between the sou~ce 2 and the ampl~fler I ~s prevented by an ~solator ~, between the be~m spl1tter 3 dnd the source 2, and the attenuator 7 ~s used to mod~fy the output of the source 2 to produce a controllable lnput s~gnal to the ampllfier 1~
The ampl1fler 1 ~s a DFB r~dge waveguide laser, 300 ~m long, w~th a coupllng coeffic~ent t~mes length of 2.4, comprislng InP w~th an InGaAsP active layer. Threshold current at room temperature ls ~7 mA and the em~sslon wavelength 1525 n~; The diode has an act~ve cross section o~ 0.4 ~ .
The source 2 ls a gratlng - tuned external cav~ty laser whlch prov~des a slngle-mode slgral~ ~hls laser ~s ~n ant~-reflect~on coated ridge wavegu1de laser, tunable ~- ln the range from 1450 to 1580 ~m tnclus1ve, aga~n compris~ng InP wlth an In6aAsP active layer.
~: The 1solator 8 ls provided by two isolatlng devfces, giY~ng together 60 dB isolation; Maxlmum coupled powers from the 50urce 2 to the amp71fler l of a few hundred can be obta~ned, as deduced from the resultant photocurrent lnduced ~n the ampl~fier 1. The beam splitter 3 comprises a slmple uncoated glass slide, and a fast PlN-pre~mp combination (not shown) provides tempor31 resolution of 100 p secs ~o allow switch~ng speed measurements by dlrectly modulating the tunable source 2.
r 1~ u~ u ~ . L, I .;, ~.
~2~3638 : 8 -A method of operat~ng the sw~tch wlll now be descr~bed, and results discussed.
The amplifler l ~s ~r~ven by d drlving current so as to produce a mater~al gafn ~n the amplif~er 1 of 0.95 times the l~s~ng threshold g~in; The source 2 1s then dr~ven to produce ~ ser~es of lnput signa1s for the ampl~f~er 1. Each Input s~gnal increases then decreases stead~ly ~n intensity, start~ng at zero, and ~s detuned, from a cav~ky resonance of the ampl1fier 1 at low ~nput ~ntenslties, by a characterlst~c amount. All the input \
si~nals have 2 wavelength shorter than the stop band of ) the dmpl~fier l;
Re~err~ng to Flgure 2, the output s~gnal ~ntensity Io ~ the ampl~ler 1 tends to show bistab~lity at low v~lues of ~nput s~gnal Int~nsity I~. However. as l;
~ncreases beyond those low values, lo becomes subst~nt~ally lnsensit~ve to I1. (As shown on F~gure 2, both slgnal intensitles lo and I~ are normallsed uslng a scaling 1ntens~ty Is;) The amount by which each Input signal Is detuned, from an output peak on ~he shore wavelengthlo~ a stop b~nd, corresponds to a slngle-pass (~e non-reflected) phase change of 0;06~, 0.13~9 0.19~, 0.2S~> ~nd 0.32~
respect~vely; It can be seen that as the detun~ng increases, the threshold ~ntensity Yalue of I; above wh1ch Io Is substant1ally ~ndependent also ~nsreases.
However9 at most the normalised threshold ~ntens~ty value ~s just below 0.01; The scal~ng intensity Is has a value of about 8 x 10~ W/cm2, wh~ch over the active cross section of the ampl~fler l corresponds to a factor 8 x 109 x 0.4 x 10 12W, or 3.2 x 10-3 W. Thus the hlghest normallsed value of I~ corresponds to an absolute value of about 30 ~W.
F R ~ 1 .J . _' I
~98~i38 g Ths results descr~bed above are relevant to an input s19nal whose 1ntensity var~es relat1vely slo~ly. It 1s ~mportant to avo1d operat~ng cond1t10ns whereunder the ampl1f1er 1 shows a sp~ked, resonant response as m1ght occur wlth a h~gh frequency slnuso1dal 1nput s19nal.
Referr1ng to F1gure 3, 1f the 1nput slgnal has a per~od of the order of the carr~er recombinat10n timQ (about 1;7 nsecs), such a spiked response 9 1s pr~d1cted. Th1s type of response is understood tostem from changes In the mater1al refract1ve 1ndex of the ampl1f1er 1 due to changes ~n the opt1cal 1nput inten~1ty;
To 1nvestlgate the preSQnCe of the sp1ked response, the output of the ampllfler 1 was calculated for four d~fferent 1nput s1gnals, an effectively steady stat¢
s1gnal (Graph ~a)), and s~nuso~dal s1gnals whose perlods corresponded to 4, 8 and 12 t1mes the carr1er recomb1nat~on t~me respect~vely ~Graphs ~b) to ~d~ ln that order), In the case of each s1nuso1dal s19nal, the sp1ked response g could be observed;
To avoid the sp1ked response 9, an osc111at1ng ~nput sign~l whose intensity 1s greater than a thresho1d value assoc1ated with a spiked response should be used; ln the case of a b~nary d~g~tal s~gnal, the 1ntenslty of the 1nput slgnal 1n the case of a "l" value should be so sel~ted, In embod~ments of the present invention, a threshold Y~lue of less than 100 ~W, and even of less than 40 ~W, should be achlevable, In the embod1ment of the present ~nvent10n descr1bed above, the period of the grat~ng selected for d~stributed feedback was second order wth regard to the wavelength of the lnput slgnal. 7~hat 1s, lt was of the order of 1/2 ~m, It is pred1cted that a lower order grat1ng would produce a stronger coupling coeff1c~entin the ampllfier ~nd so reduce the input signal intensit1es at which the deYice can be sat~sfactorily operated,
Optic~l dev1ces accordlng to embodiments of the present Invent~on hava the advantage that the rel~t~onshlp between lnput and output slgnal in~ens~t~es can show an extenslve reglon wlth1n whlch the ou~put signal fntensity Is substant1ally ~ndependent of Input slgnal ~ntenslty;
Further, because the ampll~ler ls operated in reflect~on, the devlces can offer the ablllty to use samples of hfgher llnear absorptlon.
Because the dev~ces are aet~ve rather than passlve9 there 1s also cons~derable control ava~lable over the operd~ng parameters used.
The Input-output signal Intensity relatlonshlp of an ampllf~er dccordlng to an embodiment of the present lnventlon can be used to ll~lt nolse ln an optlcal signal slnce intensity variatlDns of the Input slgnal wil~ not be reproduced. Where the signal is a blnary dlgltal s~gnal~
no~se can be llm~ted not only on "1" values of the signal, where the slynal lntens~ty l~es above the threshold value, but also on "O" values. Th~s is because lf the Gauss~an spread of the slgnal lntens~ty, assoclat~d with nolse at the ~l" value, ~s reduced, then the dec~s~on level selected to d~fferentiate between ~ and a "Oll Yalue can be ra~sed, so reduclng the effect of noise on a .,Ou value at the decis~on leYel concerned;
An optlcal llmlter according to an embodlment of the present inventlon wlll now be descrlbed, by way of example only~ w~th reference to the accompanying flgures in wh~ch:
Flgure 1 shows a schematlc representation of the llmlter In use;
Flgure 2 shows in graph form the output signal of the limlter ~n response to var~ous Input signals of different wavelengths; and Figure 3 shows the response of the lim~er to a high frequency slnusoidal Input slgnal.
F l;~ O M ~11 3 æ 0 ~ a ,- . 12 . I 'i 1 ~J ', 4 ~2~86313 Referrlng to F~gure 1, the lim~ter compr~ses a DFB
ampllfler 1 used ln reflec~lon. An opt1cal ~nput signal ls prov~ded by a tunable laser dlode so~rce 2, ~n comblnatlon w~th an attenuator 7.
A beam spl~tter 3 mou~ted between the source 2 and the ampllf~er 1 deflects a port~on of the optlcal output of the source 2 to an lnput s~gnal monltor 6, ~nd a port~on of the output o~ the ampllfler 1 to an output stgnal mon~tor 5~ Interactlon between the sou~ce 2 and the ampl~fler I ~s prevented by an ~solator ~, between the be~m spl1tter 3 dnd the source 2, and the attenuator 7 ~s used to mod~fy the output of the source 2 to produce a controllable lnput s~gnal to the ampllfier 1~
The ampl1fler 1 ~s a DFB r~dge waveguide laser, 300 ~m long, w~th a coupllng coeffic~ent t~mes length of 2.4, comprislng InP w~th an InGaAsP active layer. Threshold current at room temperature ls ~7 mA and the em~sslon wavelength 1525 n~; The diode has an act~ve cross section o~ 0.4 ~ .
The source 2 ls a gratlng - tuned external cav~ty laser whlch prov~des a slngle-mode slgral~ ~hls laser ~s ~n ant~-reflect~on coated ridge wavegu1de laser, tunable ~- ln the range from 1450 to 1580 ~m tnclus1ve, aga~n compris~ng InP wlth an In6aAsP active layer.
~: The 1solator 8 ls provided by two isolatlng devfces, giY~ng together 60 dB isolation; Maxlmum coupled powers from the 50urce 2 to the amp71fler l of a few hundred can be obta~ned, as deduced from the resultant photocurrent lnduced ~n the ampl~fier 1. The beam splitter 3 comprises a slmple uncoated glass slide, and a fast PlN-pre~mp combination (not shown) provides tempor31 resolution of 100 p secs ~o allow switch~ng speed measurements by dlrectly modulating the tunable source 2.
r 1~ u~ u ~ . L, I .;, ~.
~2~3638 : 8 -A method of operat~ng the sw~tch wlll now be descr~bed, and results discussed.
The amplifler l ~s ~r~ven by d drlving current so as to produce a mater~al gafn ~n the amplif~er 1 of 0.95 times the l~s~ng threshold g~in; The source 2 1s then dr~ven to produce ~ ser~es of lnput signa1s for the ampl~f~er 1. Each Input s~gnal increases then decreases stead~ly ~n intensity, start~ng at zero, and ~s detuned, from a cav~ky resonance of the ampl1fier 1 at low ~nput ~ntenslties, by a characterlst~c amount. All the input \
si~nals have 2 wavelength shorter than the stop band of ) the dmpl~fier l;
Re~err~ng to Flgure 2, the output s~gnal ~ntensity Io ~ the ampl~ler 1 tends to show bistab~lity at low v~lues of ~nput s~gnal Int~nsity I~. However. as l;
~ncreases beyond those low values, lo becomes subst~nt~ally lnsensit~ve to I1. (As shown on F~gure 2, both slgnal intensitles lo and I~ are normallsed uslng a scaling 1ntens~ty Is;) The amount by which each Input signal Is detuned, from an output peak on ~he shore wavelengthlo~ a stop b~nd, corresponds to a slngle-pass (~e non-reflected) phase change of 0;06~, 0.13~9 0.19~, 0.2S~> ~nd 0.32~
respect~vely; It can be seen that as the detun~ng increases, the threshold ~ntensity Yalue of I; above wh1ch Io Is substant1ally ~ndependent also ~nsreases.
However9 at most the normalised threshold ~ntens~ty value ~s just below 0.01; The scal~ng intensity Is has a value of about 8 x 10~ W/cm2, wh~ch over the active cross section of the ampl~fler l corresponds to a factor 8 x 109 x 0.4 x 10 12W, or 3.2 x 10-3 W. Thus the hlghest normallsed value of I~ corresponds to an absolute value of about 30 ~W.
F R ~ 1 .J . _' I
~98~i38 g Ths results descr~bed above are relevant to an input s19nal whose 1ntensity var~es relat1vely slo~ly. It 1s ~mportant to avo1d operat~ng cond1t10ns whereunder the ampl1f1er 1 shows a sp~ked, resonant response as m1ght occur wlth a h~gh frequency slnuso1dal 1nput s19nal.
Referr1ng to F1gure 3, 1f the 1nput slgnal has a per~od of the order of the carr~er recombinat10n timQ (about 1;7 nsecs), such a spiked response 9 1s pr~d1cted. Th1s type of response is understood tostem from changes In the mater1al refract1ve 1ndex of the ampl1f1er 1 due to changes ~n the opt1cal 1nput inten~1ty;
To 1nvestlgate the preSQnCe of the sp1ked response, the output of the ampllfler 1 was calculated for four d~fferent 1nput s1gnals, an effectively steady stat¢
s1gnal (Graph ~a)), and s~nuso~dal s1gnals whose perlods corresponded to 4, 8 and 12 t1mes the carr1er recomb1nat~on t~me respect~vely ~Graphs ~b) to ~d~ ln that order), In the case of each s1nuso1dal s19nal, the sp1ked response g could be observed;
To avoid the sp1ked response 9, an osc111at1ng ~nput sign~l whose intensity 1s greater than a thresho1d value assoc1ated with a spiked response should be used; ln the case of a b~nary d~g~tal s~gnal, the 1ntenslty of the 1nput slgnal 1n the case of a "l" value should be so sel~ted, In embod~ments of the present invention, a threshold Y~lue of less than 100 ~W, and even of less than 40 ~W, should be achlevable, In the embod1ment of the present ~nvent10n descr1bed above, the period of the grat~ng selected for d~stributed feedback was second order wth regard to the wavelength of the lnput slgnal. 7~hat 1s, lt was of the order of 1/2 ~m, It is pred1cted that a lower order grat1ng would produce a stronger coupling coeff1c~entin the ampllfier ~nd so reduce the input signal intensit1es at which the deYice can be sat~sfactorily operated,
Claims (18)
1. An optical signal processing device comprising a semiconductor DFB laser amplifier operated in reflection, means for coupling an optical input signal to the amplifier, the signal being detuned from an output signal peak to the short wavelength side of the wavelength range relevant to an amplifier stop band, and means for applying a driving current to the amplifier of less than the lasing threshold driving current for the amplifier, the input signal reaching for at least part of the time an intensity greater than a threshold value above which the output signal intensity of the amplifier is substantially independent of the input signal intensity.
2. An optical signal processing device according to claim 1 wherein the input signal is a binary digital signal whose intensity for a "1" value is greater than the threshold value.
3. An optical signal processing device according to claim 1 wherein the threshold value is less than 100 µW.
4. An optical signal processing device according to claim 3 wherein the threshold value is less than 40 µW.
5. An optical signal processing device according to claim 1 wherein the amplifier is provided with a grating for distributed feedback, whose period is of a low order with reference to the wavelength of the input signal.
6. An optical signal processing device according to claim 5 wherein the period of the grating is of a third order or less with reference to the wavelength of the input signal.
7. An optical limiter comprising an optical signal processing device according to either claims 1 or 2.
8. A noise filter comprising an optical signal processing device according to any one of claims 1 or 2.
9. An optical signal processing device comprising:
a semiconductor DFB laser amplifier operated in reflection for producing an optical output signal, means for coupling an optical input signal to the amplifier, said input signal having an optical frequency disposed to the short wavelength side of an amplifier stop band, and means for applying a driving current to the amplifier of less than a lasing threshold for the amplifier, said input signal reaching, for at least part of the time, an intensity greater than a threshold value above which the intensity of said amplifier output signal is substantially independent of said input signal intensity.
a semiconductor DFB laser amplifier operated in reflection for producing an optical output signal, means for coupling an optical input signal to the amplifier, said input signal having an optical frequency disposed to the short wavelength side of an amplifier stop band, and means for applying a driving current to the amplifier of less than a lasing threshold for the amplifier, said input signal reaching, for at least part of the time, an intensity greater than a threshold value above which the intensity of said amplifier output signal is substantially independent of said input signal intensity.
10. An optical signal processing device according to claim 9 arranged to provide an optical signal limiter.
11. An optical signal processing device according to claim 9 arranged to provide an optical signal noise filter.
12. An optical signal processing device comprising:
a DFB laser amplifier, operated in reflection, having a frequency band sensitive response disposed to the short wavelength side of an amplifier stop band for accepting an input optical signal of varying intensity and producing, in response, an output optical signal;
means for coupling said input optical signal to said laser amplifier at an optical frequency located to said short wavelength side of the amplifier stop band; and means for applying a driving current to said amplifier below its lasing threshold thereby causing said output optical signal to have an intensity substantially independent of the input optical signal intensity when said input optical signal intensity is above a predetermined threshold value.
a DFB laser amplifier, operated in reflection, having a frequency band sensitive response disposed to the short wavelength side of an amplifier stop band for accepting an input optical signal of varying intensity and producing, in response, an output optical signal;
means for coupling said input optical signal to said laser amplifier at an optical frequency located to said short wavelength side of the amplifier stop band; and means for applying a driving current to said amplifier below its lasing threshold thereby causing said output optical signal to have an intensity substantially independent of the input optical signal intensity when said input optical signal intensity is above a predetermined threshold value.
13. An optical signal processing method for operating a semiconductor DFB laser amplifier in reflection to produce an optical output signal having an intensity which is substantially independent of an input optical signal intensity above a predetermined threshold, said method comprising:
coupling an optical input signal to the amplifier, said input signal having an optical frequency disposed to the short wavelength side of an amplifier stop band, and applying a driving current to the amplifier of less than a lasing threshold for the amplifier, said input signal reaching, for at least part of the time, an intensity greater than a threshold value above which the intensity of said amplifier output signal is substantially independent of said input signal intensity.
coupling an optical input signal to the amplifier, said input signal having an optical frequency disposed to the short wavelength side of an amplifier stop band, and applying a driving current to the amplifier of less than a lasing threshold for the amplifier, said input signal reaching, for at least part of the time, an intensity greater than a threshold value above which the intensity of said amplifier output signal is substantially independent of said input signal intensity.
14. An optical signal processing method for operating a DFB laser amplifier in reflection, the amplifier having a frequency band sensitive response disposed to the short wavelength side of an amplifier stop band to accept an input optical signal of varying intensity and to produce, in response, an output optical signal having a controlled intensity variation, said method comprising:
coupling said input optical signal to said laser amplifier at an optical frequency located to said short wavelength side of said amplifier stop band; and applying a driving current to said amplifier below its lasing threshold thereby causing said output optical signal to have an intensity substantially independent of the input optical signal intensity when said input optical signal intensity is above a predetermined threshold value.
coupling said input optical signal to said laser amplifier at an optical frequency located to said short wavelength side of said amplifier stop band; and applying a driving current to said amplifier below its lasing threshold thereby causing said output optical signal to have an intensity substantially independent of the input optical signal intensity when said input optical signal intensity is above a predetermined threshold value.
15. An optical processing device according to claim 12 arranged to provide an optical signal limiter.
16. An optical processing device according to claim 12 arranged to provide an optical signal noise filter.
17. An optical signal processing method as claimed in claim 13 or 14, applied to provide an optical signal limiter.
18. An optical signal processing method as claimed in claim 13 or 14, applied to provide an optical signal noise filter.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB868629873A GB8629873D0 (en) | 1986-12-15 | 1986-12-15 | Optical signal processing device |
| GB8629873 | 1986-12-15 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1298638C true CA1298638C (en) | 1992-04-07 |
Family
ID=10609004
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000554367A Expired - Fee Related CA1298638C (en) | 1986-12-15 | 1987-12-15 | Optical signal processing device |
Country Status (8)
| Country | Link |
|---|---|
| US (1) | US4962502A (en) |
| EP (1) | EP0275663B1 (en) |
| JP (1) | JPH01502614A (en) |
| AT (1) | ATE104065T1 (en) |
| CA (1) | CA1298638C (en) |
| DE (1) | DE3789542T2 (en) |
| GB (1) | GB8629873D0 (en) |
| WO (1) | WO1988004790A1 (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5103455A (en) * | 1990-05-09 | 1992-04-07 | Gte Laboratories Incorporated | Monolithically integrated semiconductor optical preamplifier |
| FR2682781B1 (en) * | 1991-10-16 | 1993-11-26 | France Telecom | LOGIC FUNCTION GENERATOR USING OPTICAL TRANSPOSITION IN A LASER DIODE WITH DISTRIBUTED FEEDBACK. |
| US5383199A (en) * | 1992-07-02 | 1995-01-17 | Advanced Interventional Systems, Inc. | Apparatus and method for optically controlling the output energy of a pulsed laser source |
| DK0901713T3 (en) * | 1996-05-30 | 2000-09-25 | Deutsche Telekom Ag | Generator for generating a low frequency noise-low signal |
| US6160504A (en) * | 1999-06-30 | 2000-12-12 | Trw Inc. | Repetitive absorptive thresholding optical quantizer |
| US8100893B2 (en) * | 2007-11-28 | 2012-01-24 | The Spectranetics Corporation | Laser catheter calibrator |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3724926A (en) * | 1971-08-09 | 1973-04-03 | Bell Telephone Labor Inc | Optical pulse modulator |
| US4215323A (en) * | 1978-01-13 | 1980-07-29 | Bell Telephone Laboratories, Incorporated | Instability and regenerative pulsation in optical cavities |
| US4751710A (en) * | 1984-07-26 | 1988-06-14 | Nec Corporation | Semiconductor laser device |
| JPS61201222A (en) * | 1985-03-04 | 1986-09-05 | Hitachi Ltd | Light pulse amplifying and shaping device |
| GB8629871D0 (en) * | 1986-12-15 | 1987-01-28 | British Telecomm | Optical switch |
-
1986
- 1986-12-15 GB GB868629873A patent/GB8629873D0/en active Pending
-
1987
- 1987-12-14 US US07/230,835 patent/US4962502A/en not_active Expired - Lifetime
- 1987-12-14 AT AT87310985T patent/ATE104065T1/en not_active IP Right Cessation
- 1987-12-14 WO PCT/GB1987/000904 patent/WO1988004790A1/en unknown
- 1987-12-14 EP EP87310985A patent/EP0275663B1/en not_active Expired - Lifetime
- 1987-12-14 DE DE3789542T patent/DE3789542T2/en not_active Expired - Fee Related
- 1987-12-14 JP JP63500403A patent/JPH01502614A/en active Pending
- 1987-12-15 CA CA000554367A patent/CA1298638C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| GB8629873D0 (en) | 1987-01-28 |
| ATE104065T1 (en) | 1994-04-15 |
| US4962502A (en) | 1990-10-09 |
| JPH01502614A (en) | 1989-09-07 |
| EP0275663B1 (en) | 1994-04-06 |
| EP0275663A1 (en) | 1988-07-27 |
| DE3789542D1 (en) | 1994-05-11 |
| WO1988004790A1 (en) | 1988-06-30 |
| DE3789542T2 (en) | 1994-07-14 |
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