Tunable lasers handbook phần 9 pptx

384 Paul Zorabedian 7.1.1.5 Camera Lenses There are at least three published reports on the use of camera lenses as col- limators in ECLs. Heckscher and Rossi [57] reported the use of a TV camera lens for intracavity collimation of a Littrow grating cavity, but gave no indication of the feedback strength obtained. Sommers [58] evaluated several camera lenses from f10.99 (25-mm focal length) to f12.0 (50-mm focal length). The lenses gave only about 1% feedback when used with a grating, and it was con- cluded that spherical aberration was responsible for the poor performance since the lenses were not used in their intended geometry. Fleming and Mooradian successfully employed camera lenses in an ECL [38]. They used 50-mm focal length,fll.4 seven-element lenses. All air-glass surfaces were AR coated. 7.1.1.6 Ball Lenses Glass spheres can be used to couple the gain medium to waveguide or fiber- pigtailed external filters. However, the spherical aberrations are too great to be useful for collimation in bulk optic cavities. 7.1.1.7 Lensed Fiber Lensed optical fiber [59] can be used to couple the gain medium to fiber- pigtailed external cavities. However. this method requires the fiber to be in very close proximity to the facet, which gives rise to the danger of facet damage. There is also a very high sensitivity of the coupling loss to lateral misalignment. 7.7.2 Optics for Beam Expansion and Shaping 7.1.2.1 Cylindrical Lenses A cylindrical lens can be used in an ECL [60] to form a line illumination on a diffraction grating. This implements a degenerate resonator in one dimension and provides a high degree of angular misalignment tolerance while maintaining high spectral selectivity. Critical to the success of this technique is the fact that the cylinder axis can be inclined with respect to the optical axis at a large angle to match the grating angle of incidence without introducing a large amount of spherical aberration. This is because the cylinder lens has no power in this plane and appears to be a tilted plate. 7.1.2.2 Prisms The use of prism beam expanders allows the use of a compact, high-resolution grating-tuned extended-cavity laser [61]. A particularly useful geometry is when the apex angle 8, is cut so that 8, = 90" - tan-' (11) . (44) where ri is the index of refraction of the prism material. For this choice of apex angle, the output beam is normal to the exit face of the prism (which is the 8 Tunable External-Cavity Semiconductor Lmers 385 condition of maximum expansion) when the angle of incidence equals the Brew- ster angle. The magnification of each prism is then equal to the index of refrac- tion of the prism material, that is, M = 17. 7.2 Tunable Filters The ideal filter for an ECL has a bandwidth that is less than the axial mode spacing of the cavity and has 0-dB insertion loss at its peak. No real filter is ideal, but a number of different types of wavelength-selective elements have been used to tune external cavity lasers. The filters are grouped according to whether they are actuated by mechanical means (e.g., have moving parts) or electronically (no moving parts). 7.2. 7 Mechanically Tuned Filters 7.2.1.1 Diffraction Gratings 7.2.1.1.1 Types of Gratings Diffraction gratings are the most common type of filter used in ECLs and have arguably the best optical performance. A diffraction grating consists of a large number of regularly spaced grooves on a substrate. The distance between adjacent grooves is called the pitch. If the underlying substrate is reflective. then we have a I;?jection gl-atiizg [Fig. 18(a)]. If the substrate is transmissive, then the device is said to be a tl-ansmissiorz gmtiizg [Fig. 18(b)]. Diffraction gratings are also classified by the way in which they are manu- factured. When the grooves are created by scribing with a ruling engine, the device produced is a ruled mastel- grating. Relatively few masters are produced, and these are rarely sold. The groove pattern of the master can be faithfully Transferred by a contact process to a number of replica gratings, which are then made available commercially (e.g by Milton Roy). Diffraction grating groove patterns are also generated by exposing photo- resist with the fringe pattern created bj two interfering beams of laser light, Such gratings are called holographic and are also sold commercially (e.g., by American Holographic). 7.2.1.1.2 Principle of Operation When a beam of light is incident on a grating, each groove generates a dif- fracted wavelet. For each wavelength component in the incident beam, the con- structive interference of the diffracted components from each groove occurs at a unique set of discrete directions called the diffraction oi-del-s of the grating. 7.2.1.1.3 The Grating Equation grating equation: The geometry of the diffraction pattern from a grating is governed by the 386 Paul Zorabedian a [ sin oi + sin cp,) = n7~ , (45) where a is the groove spacing (pitch). is the incident angle, 'p, is the diffracted angle of the m'th order, and n7 is the order of diffraction. The diffracted light is dispersed according to its spectral content. with different wavelengths appearing at different angles. Differentiating the grating equation gives the angular disper- sion D, which describes how much the diffraction angle changes as the wave- length varies: Diffraction gratings are usually used in first order in ECLs, that is. with ni = 1. The zeroth-order (specular reflection) beam is sometimes used for output coupling. The wavelength resolution of a grating-tuned external cavity is determined by the angular dispersion multiplied by the acceptance angle for coupling back into the gain medium active region. The angular dispersion can therefore be used FIGURE 1 8 (reproduced with permission from Palmer [62]). Types of plane diffraction gratings. (a) Reflection grating. (b) Transmission grating 8 Tunable External-Cavity Semiconductor lasers 387 as a figure of merit, but it must be remembered that the parameter of ultimate importance is the grating resolution divided by the axial mode spacing of the external cavity. (For a detailed description of multiple-prism grating dispersion. see Chapier 2.) 7.2.1.1.4 Common Mountings Diffraction gratings in external cavity lasers combine the functions of the fil- ter and external mirror. In extended cavities, the light from the grating must be retroreflected back into the gain medium. Two common retroreflecting mounting geometries for diffraction gratings in extended-cavity lasers are the autocollima- tion (Littrow) configuration and the grazing-incidence (GI) configuration. 7.2.1.1.4.1 Littrow Moztnting In the Littrow configuration [Fig. 19(a)], [he angles of incidence and diffraction are equal: Oj = 'pl. The grating equation becomes In this case the angular dispersion of the retroreflected beam is identical to that of the diffracted beam and is given by A typical angle of incidence for the Littrow configuration is Oi - 50". 7.2.1.1.4.2 Grazing-Zncidence Mounting In the grazing-incidence con- figuration (Fig. 19b). the intracavity beam makes two passes at the grating. The diffracted light from the second pass is a retroflection of the incident light from FIGURE 1 c' Diffraction grating mountings. (a) Littrou. (b) Grazing incidence. 388 Paul Zorabedian the first pass. Therefore, the angular dispersion of the retroreflected light is twice that of the light diffracted on one pass: The dispersion of the grazing-incidence configuration is therefore twice that of the Littrow configuration for the same angle of incidence. In addition, the grazing- incidence configuration is typically used with a much higher angle of incidence, for example, 8, - 85". 7.2.1.1.5 Grating Efficieizcy 7.2.1.1.5.1 Blazed Gratings Blazing refers to an enhancement in effi- ciency that is obtained at a particular wavelength when the grooves on the grat- ing surface have a triangular shape. A simple explanation for this effect is that when the specular reflection from the top surface of each groove coincides with the direction of diffraction, the reflections reinforce the diffraction effect and the efficiency is maximized. The wavelength h, at which this reinforcement occurs is called the "blaze wavelength." The angle 8, of the top surface of the groove with respect to the macroscopic surface of the grating is called the "blaze angle." The terminology derives from the observation that a grating will light up or "blaze" when viewed at the correct angle. The blaze angle of ruled gratings is defined during the process of ruling the master grating and is transferred to the replica. The simplest type of holographic grating has a sinusoidal shape. However, after interferometric recording, the grooves of holographic gratings can be shaped to approximate blazing by an ion- beam milling process. In a Littrow mounting the blaze condition is satisfied when the tops of the grooves are perpendicular to the incident beam. The diffraction efficiency rises as the angle of incidence is increased up to -8, and falls thereafter. This simple description is only valid for low blaze angles (up to -10'). Working near 1, for small blaze angles implies a small diffraction angle as well, so that k<a. This is the regime of validity for scalar diffraction theory, in which the diffraction effi- ciency is nearly independent of polarization. 7.2.1.1.5.2 Polarization Effects To obtain greater angular dispersion it is necessary to use larger blaze and diffraction angles, which implies IL - a. This is the regime of vector diffraction theory in which polarization effects become sig- nificant. For blaze angles above -lo", the diffraction efficiency strongly depends on the orientation of optical polarization with respect to the direction of the grooves. A particularly useful regime for tuning ECLs is the range of blaze angles from about 22" to 38". For this regime, there is a broad plateau of high efficiency for €Il > 8, when the incident polarization is perpendicular to the 8 Tunable External-Cavity Semiconductor Lasers 389 direction of the grooves on the grating (Fig. 20). The reader who desires further details on i:he subject of grating efficiency and polarization effects is advised to consult the excellent material in [62]. 7.2.1.1.6 Wavelength Resolution The wavelength resolution is obtained by dividing the angular spread of the beam waist at the grating (waist divergence) by the angular dispersion. The waist divergence of a Gaussian beam of radius vi!" is given by The wavelength resolutions for the Littrow and grazing-incidence cases are, respectively: (52 j It is useful to relate grating resolution to thefilled depth of the grating. The filled depth is the projection of the illuminated region of the grating onto the optical axis of the cavity. The filled depth Lw is given by 0 r~i~i~r~i~ir~~i First-Order Littrow Diffraction Angle 0' 10' 20' 30' 40' 50' 60' 90' FIGURE 20 Efficiency versus angle of incidence for Littrow grating (from Palmer [62]). 390 Paul Zorabedian For the Littrow geometry, the grating resolution can be expressed in terms of the filled depth as (54) - XL, . RVHXI(Lil1rouI - For the grazing-incidence geometry. the resolution is (55) h’ KL, . ‘LWH,,,,,, = 2 In terms of optical frequency. the grating reflectance function for a Gaussian beam is given by [63] where the band width is given by C AVRVHkl = ~ KL,. (57) 7.2.1.2 Distributed Bragg Reflector 7.2.1.2.1 Principle of Operation Periodic modulation of the index of refraction along the length of an optical waveguide results in a structure known as a distributed Bragg reflector. The reflection is maximized at a wavelength for which the period of the modulation is equal to h/3. If the modulation period can be varied, then the reflected wave- length can be tuned. 7.2.1.2.2 Embodiment in Optical Fiber A variable-wavelength distributed Bragg reflector for single-mode optical fiber has been realized in the following form [64]. An optical fiber was placed in a groove in a fused silica substrate. The substrate was then polished until part of the cladding of the fiber was removed. On a separate substrate, a fan-shaped grating consisting of slowly diverging lines of sputtered amorphous silicon was fabricated. The grating was placed face-down on the side-polished fiber with a small amount of index-matching oil between the substrates. The grating then was able to interact with the evanescent field in the fiber. The grating substrate was able to slide over the fiber substrate, thus changing the pitch of the grating 8 Tunable External-Cavity Semiconductor Losers 391 that was coupled to the fiber evanescent field. In this way, a fiber reflective grat- ing was obtained that had a reflectance of -60 to 80% for 1280 nm < h < 1340 nm. The grating FWHM was between 0.7 to 1.2 nm. 7.2.1.3 Fabrg-Perot Etalon 7.2.1.3.1 Principle oJf Operation The filtering effect of the Fabry-Perot etalon utilizes the interference fringes produced in the transmitted light after multiple reflections between two highly reflective mirrors [65]. The Fabry-Perot etalon has periodic transmission peaks at wavelengths that satisfy the relation 2nd cos 0 = nzh . (58) where d is the mirror spacing. 12 is the index of refraction of the space between the mirrors, 0 is the angle of incidence, and m is an integer. Tuning can be accom- plished by changing the mirror separation or by varying the angle of incidence. 7.2.1.3.2 Resolution The ratio of the wavelength of a fringe peak to the FWHM of the peak of a Fabry-Perot etalon is called the chr-omnnc I-esohing pow'en The chromatic resolving power is given by where I' is the amplitude reflectance of the mirrors. 7.2.1.3.3 Free Spectral Range u avelength spacing between maxima is given by the free spectral range. For a typical air-spaced or solid etalon, d is equal to a few millimeters. The For example. for h = 1300 nm. d = 1 mm, and tz = 1.5, the free spectral range is 0.56 nm, 7.2.1.3.4 Finesse The spacing between orders relative to the width of a single order is given by the finesse -6 The finesse is defined as 392 Paul Zorabedian With special mirror coating technology. the finesse of an etalon can be as high as =10,000, but a finesse of a few hundred is more typically achieved with conven- tional coatings. 7.2.1.4 Interference Filter A bandpass interference filter is a multilayer thin-film device [66]. The sim- plest type is really a Fabry-Perot etalon with d - h. If the thickness of an etalon is made very small, the orders will be widely separated. This is done by evapo- rating dielectric-stack mirrors, separated by a half-wave spacer layer, in a contin- uous coating run on a substrate. Multiple reflector pairs (called cavities) can be deposited to steepen the passband. Additional metallic-layer blocking stmctures deposited on another plate are used to eliminate adjacent transmission orders. The plates are assembled in a sandwich that protects the deposited films. Inter- ference filters can be made with FWHM bandwidths 2 nm or less in the near infrared and less than 1 nm in the visible. The peak transmittance can be made as high as 50 to 70%. The interference filter is tuned by tilting it in the incident beam. For small angles (up to 5 to loo). the wavelength of peak transmittance is given by where 8 is the angle of incidence, izo is the refractive index of the external medium, and lie is the effective refractive index of the spacer. 7.2.2 Electronically Controlled Filters 7.2.2.1 Birefringent Filter There are several forms of the birefringent filter [67,68]. They can be tuned either mechanically or electronically, with electronic tuning being the preferred means. The basic birefringent filter is called a Lyotfiltel- and comprises an alter- nating stack of N uniaxial birefringent plates separated by polarizers. The thick- nesses of the plates vary in a geometrical progression d, 2d, 4d, . . . . 2”-’d. The transmission axes of the polarizers are all aligned. The light propagates in a direction perpendicular to the c axis of each of the plates. Transmission through each segment (plate plus polarizer) will vary sinusoidally, with maxima at wave- lengths for which the retardation of the plate is a multiple of 2x. For a plate of thickness d, the free spectral range Ah,,, between successive maxima is approx- imated by h 1 Ah FSR - (dAn/dh - An/h) 8 Tunable External-Cavity Semiconductor Lasers 393 For each segment, the separation between transmission maxima and the FWNM of one of the maxima is inversely proportional to the plate thickness. Thus, the resulting ti-ansmission spectrum for the entire stack will consist of narrow bands having the FWHM of the thickest plate and separated by the free spectral range of the thinnest plate. Electronically tuned birefringent filters can be realized using liquid crystal cells as the birefringent plates [69,70]. The electro-optic effect can also be used, either in bulk crystals [7 I] or in birefringent lithium nio- bare waveguides [72], 7.2.2.2 Acousto-Optic Tunable Filter 7.2.2.2.1 Principle of Operation The acousto-optic tunable filter (AOTF) operates on the principle of aniso- tropic BrBgg diffraction in a birefringent crystal. A piezoelectric transducer is bonded to a crystal. When the transducer is driven with an rf signal, a traveling acoustic wave is generated. The acoustic w'ave produces a moving refractive index grating (phase grating) in the crystal via the elasto-optic effect. Under the proper conditions, the AOTF couples a portion of the energy in a linearly polarized inci- dent beam of light into an orthogonally polarized output beam. The interaction must satisfy the phase-matching condition k - kl & k,, where k,, k8 and k, are, respectively, the momentum vectors of the incident, diffracted, and acoustic waves (Fig, 31). The AOTF is designed so that, for a given acoustic frequency. only a narrow range of optical frequencies will satisfy the phase-matching dT P FIGURE 2 1 an .40TF. Index ellipsoids and optical and acoustic k vectors illustrating phase matching in [...]... zeroth-order grating reflection for output coupling (Fig 28) Littrow GRIN rod lens collimator, intracavit) silicon prism beam expanders (Fig 29) Littrow Tapered-waveguide gain chip 1-W air beam output (Fig 30) Littrow [60]( 199 01 [85]( 199 11 [61] ( 199 2) [13]( 199 3) 400 Paul Zorabedian TUNING AXIS DIFFRACTION c , 3 CYLINDRICAL OUTPUT COLLIMP.TIN6 OBJECTIVE FIGURE 27 Alignment stabilization of a Littrowgrating... nm at around 1.3 pm have been achieved (Fig 24) 7.2.2.2.3.3 Diffraction Efficiency The diffraction efficiency is given by 396 Paul Zorabedian ! 20 Wavelength (nm) FIGURE 24 Transmission spectrum of an AOTF driven at 89. 1 39 MHz (Reproduced with permission from Zorabedian [46] 0 199 5 IEEE.) where I, and Idare, respectively, the incident and diffracted intensity, Pa is the acoustic power, h and 1.1’ are,... Schremer and Tang [lo@.0 199 0 IEEE.) 412 Paul Zorabedian v ARBITRARY PIVOT PIVOT / ARCOATING I , a - # * GRATING I L1 tan01 I i;.l I e 4 OPTIMUM PIVOT L3 FIGURE 37 Location of optimum pivot point in phase-continuous tuned prating extendedcavity laser (Reproduced with permission from Trutna and Stokes [112] @ 199 3 IEEE.) 1 1 CHARACTERIZATION METHODS F R EXTERNAL-CAVITY LASERS O To design and... could be generated without the differential-delay interferometer 8 Tunable External-CavitySemiconductor Lasers ~~~~i~~ Laser Diode 4 19 DirectionalCoupler PZT Fiber Delay Line Laser Output FlGuRE 4 1 Interferometric apparatus for mode-hopping suppression in an extended-cavity laser (Reproduced with permission from Ohtsu et a! [120] 0 198 9 IEEE.) Open-loop control is an alternative to a closed-loop servo... with two chirpcompensating AOTFs (Reproduced with permission from Zorabedian [46] 0 199 5 IEEE.) 408 Paul Zorabedian \ lo Fiber ?? Matching Network Optical Amplifier AR Coatings AOTF #2 Matching FIGURE 35 Ring ECL tuned with two chirp-compensating AOTFs (Reproduced with Fermission from Zorabedian [46] 0 199 5 IEEE.) In practice, the grating resolution will ultimately be limited by the width... u DIFFRACTION 1 -3 ALIGNMENT AXIS AR CGA.TlNG OUTPUT FlGURE 26 Trutna [60].) COLLIMATING OBJECTIVE Standard Littrow-grating ECL (Reproduced with permission from Zorabedian and 8 Tunable External-CavitySemiconductor Lasers 399 8 I 3 Grating-Tuned Ring External Cavities The first report of a grating-tuned ring ECL operating in strong-feedback mode was by Bogatov and coworkers [86] The active element was... conversion [72] Metal directly overlaying the waveguide without a buffer layer provided a strong differential attenuation 8 Tunable External-Cavity Semiconductor Lasers 405 €or the unconverted TM-polarized light [96 ] The tuning rate of the filter was -0.05 nm/Y and its FWHM bandwidth was 1.2 nm [97 ] The extended cavity did not provide much feedback, as demonstrated by the fact that its threshold current wais... oscillation was not obtained Peng and Su [88] described a 1300-nm free-space ring ECL comprising a 1000-pm-long tilted-stripe amplifier, a 600 groove/mm grating, and an optical 8 Tunable External-CavitySemiconductor lasers 401 FIGURE 29 Littrowgrating extended-cavity laser with a GRIN rod lens collimator and intracavity silicon prism beam expanders (Reproduced with permission from Zorabedizn [61] ) AR coating... ECLs Some examples follow Kahn and coworkers [ 89] constructed a pair of high-stability etaloncontrolled ECLs For this design, the gain element was a 400-pm-long dualelectrode buried-heterostructure InGaAsP laser diode with one HR and one AR facet The extended cavity comprised a 100-pm air gap etalon and an output 8 Tunable External-Cavity Semiconductor Lasers 403 INTERFERENCE FILTER / OUTP / TUNING... tune a fiber ring ECL [91 ] The filter lhad a 0.3-nm bandwidth and a 30-nm free spectral range The gain 404 Paul Zorabedian medium was a 400-ym-long, 1.5-ym semiconductor amplifier with a reflectance of -10-4 for the AR coating on each facet The 18-m fiber loop contained two inline optical isolators and a polarization controller Output was obtained with a 90 :lO directional coupler (90 % feedback 10% output) . 29) Tapered-waveguide gain chip. 1-W air beam output (Fig. 30) [81] ( 197 2) [82] ( 198 5) [I61 ( 198 5) [83] ( 198 71 [81] ( 198 81 [60] ( 199 01 [85] ( 199 11 [61] ( 199 2) [13] ( 199 3). is given by 396 Paul Zorabedian !2.0 Wavelength (nm) FIGURE 24 mission from Zorabedian [46]. 0 199 5 IEEE.) Transmission spectrum of an AOTF driven at 89. 1 39 MHz. (Reproduced. (Reproduced with permission from Zorabedian and -3 ALIGNMENT AXIS 8 Tunable External-Cavity Semiconductor Lasers 399 8. I .3 Grating-Tuned Ring External Cavities The first report of