Electronic wavelength tuning with semiconductor integrated etalon interference lasersArsam Antreasyan and Shyh Wang Citation: Applied Physics Letters 43, 530 (1983); doi: 10.1063/1.94427 View online: http://dx.doi.org/10.1063/1.94427 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/43/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Tunable optofluidic dye laser with integrated airgap etalon AIP Conf. Proc. 1288, 18 (2010); 10.1063/1.3521357 Frequency locking and wavelength tuning of nanosecond pulsed broad-area semiconductor lasers Appl. Phys. Lett. 84, 4265 (2004); 10.1063/1.1758782 Measurement of the zero-bias electron transmittance as a function of energy for half- and quarter-electron-wavelength semiconductor quantum-interference filters Appl. Phys. Lett. 72, 374 (1998); 10.1063/1.120741 Differential wavelength meter for laser tuning Rev. Sci. Instrum. 68, 1648 (1997); 10.1063/1.1147971 Semiconductor integrated étalon interference laser with a curved resonator Appl. Phys. Lett. 42, 562 (1983); 10.1063/1.94028

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Electronic wavelength tuning with semiconductor integrated etalon interference lasers

Arsam Antreasyan and Shyh Wang Department of Electrical Engineering and Computer Sciences and the Electronics Research Laboratory, University of California, Berkeley, California 94720

(Received 6 June 1983; accepted for publication 28 June 1983)

A novel method for broadband quasicontinuous wavelength tuning in GaAs-GaAIAs semiconductor lasers is reported. The wavelenth tuning experiment is performed with an interferometric laser consisting of a resonator with curved and straight segments. By separately pumping different segments of the laser the output wavelength can be tuned over a wide range. The outstanding features of the device are (I) very stable single longitudinal mode and stable transverse mode operation, and (2) a wavelength tuning range of as much as 90 A. PACS numbers: 42.55.Px, 42.60.0a

Semiconductor lasers are considered a primary source of coherent radiation for present optical fiber communica­tion systems. Recently, there has been significant improve­ment in the temperature stabilization of the longitudinal mode of semiconductor lasers by using interferometric re­sonators. l

--1 Also, there has been considerable interest in wavelength-tunable semiconductor lasers since such tunable diode lasers can be used for optical wavelength modulation as well as wavelength-division multiplexing which would greatly improve the performance of the present optical fiber communication systems. Several approaches have been re­ported, for example, by direct modulation of the injection current' and by utilization of the electro-optic effecth

; but direct modulation of the injection current produces an ex­tremely small tuning range and the utilization of the electro­optic effect requires very complex integrated cavity struc­tures. Recently, direct frequency modulation over a wide range has been obtained in a cleaved-coupled-cavity semi­conductor laser. 7 This method uses two standard Fabry­Perot cavity 1.3-fLm wavelength InGaAsP crescent laser di­odes which are very closely coupled to form a two-cavity resonator. This structure, however, may be difficult to fabri­cate since it requires an extremely sensitive alignment of the two lasers.

In this letter we report a novel structure to obtain a very wide wavelength-tuning range, using a separately pumped interferometric semiconductor laser cavity, as shown in Figs. I(a) and l(b). The structure consists of two straight segments (L I and L,) and a quarter-ring segment (L.,). La­teral optical confinement is provided by the fabrication of a buried heterostructure (BH) type laser cavity. The fabrica­tion procedures of the laser and its interferometric proper­ties were described in detail in Ref. 4. Separate pumping is accomplished by etching a 20-fL m-wide stripe into the p-con­tact metallization (Cr/ Au), using gold and chromium et­chants consecutively. After cleaving the wafer into individ­ual devices the structure, as shown in Fig. l(b), is obtained. The resistance between the two contact pads is measured to be around 200--300 fl. In accordance with Ref. 4 we call this laser a separately pumped integrated etalon interference (SPIEl) laser for easy reference. According to Fig. lib) we obtain two coupled cavities with different lengths L I and

L: + L, separated by an unpumped region of20l1m length. The coupling between the two separately pumped regions is caused by internal reflection and lateral mode conversIOn at the junction discontinuity between straight and curved waveguides.

The basic working principle of the laser is illustrated in Fig. 2 where we distinguish between two separate cases. The longitudinal mode spacing in each cavity depends on the refractive index and the length of the active region. Under separate pumping, the carrier concentration in each segment is no longer fixed above the laser threshold even though the overall gain of the laser is still held constant by the total cavity loss. Since the refractive index is dependent on carrier concentration, the optical path length of each cavity changes with the individual current. Figures 2(a) and 2(b) consist of three different spectra, where the first one indicates the lon­gitudinal mode spectrum of the L I cavity, and the second represents the longitudinal modes oftheL2 + L, cavity. As a result of the two cavities having different optical path lengths there will be coincidence-resonant modes (CRM) as indicated in the third spectrum with solid arrows. If we de­fine L = L I + (L2 + L,), J.L = L I - (L2 + L ,i. and q = L / J.L, then the case of Fig. 2(a) corresponds to a relatively small q, while Fig. 21b) corresponds to a relatively large q. In the first case with small q, as we keep the current II into the L I cavity constant and vary the current I: into the L: + L,

lal

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Ibl 20 jJ m

FIG. 1. (al Structure of the laser without Ohmic contact separation with R = 100 11m; (bl the SPIEl laser with Ohmic separation between Ihe L I cavity and the L, - L, cavity.

530 Appl. Phys. Lett. 43 (6), 15 September 1983 0003-6951/83/180530-03$01.00 @ 1983 American Institute of Physics 530

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!

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FIG. 2. Baisc working principle of the SPIEl laser illustrated for two sepa' rate cases with the longitudinal modes of the L, cavity. the longitudinal modes of the L, + L, cavity prior to a spectrum shift (solid arrows) and after a spectrum shift (dotted arrows). and the corresponding CRM's (lfthe two­cavity resonator prior to a shift Isolid arrow) and after a shift (dotted arrow).

cavity the longitudinal mode spectrum is shifted (indicated as dotted arrows). However, as a result of having a small q the spacing between the new CRM (indicated as doted ar­row) and the former CRM (solid arrow) corresponds to an integral multiple of the longitudinal mode spacing of the L I

cavity. Now we consider the second case with a relatively large q. By varying Ie the mode spectrum of the Le + L1 cavity is shifted but as a result of having a large q the CRM changes to the next nearest longitudinal mode of the L I cav­ity. Considering these two cases we conclude that there can­not be a strictly continuous wavelength tuning in a coupled cavity structure. However, by cleaving the devices accord­ingly to obtain large values of q we will be able to tune the wavelength of the coupled cavity to every possible longitudi­nal mode of the L I cavity within the gain bandwidth of the active medium. In addition, we have the ability to vary II while keeping Ie constant which enables us to tune the laser to even more wavelengths in between the longitudinal modes of the L I cavity. In this way, we should be able to stitch together overlapping regions of continuous wavelength tun­ing by properly adjusting the two currents. In conclusion, having a large value of q enables us to tune the laser to many different wavelengths within a certain range; thus, we denote this mode of operation as quasicontinuous wavelength tun­Ing.

The SPIEl lasers considered in the following experi­mental results have a curvature radius of 100 J.lm. The lasing spectra are measured at the cleaved facet of the Le + L1 cav­ity. Figure 3(a) shows lasing spectra of a laser with L I = 300 J.lm and Le + L1 = 210 J.lm corresponding to a value of

531 Appl. Phys. Lett., Vol. 43, No.6, 15 September 1983

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8530 'hi 8769 8730

WAVE LENGTH (A)

FIG . .1. Lasing spectra for two different lasers with (a) L, .lOO Jim.

L, + L, = 2lO f lm. and (b) L, = 2251Im. L + L ,= 210llm at various cur­rent levels.

q = 5.6 where we have a relatively small number of tunable wavelengths. This represents the case described in Fig. 2(a); thus, we observed a relatively large mode hopping within a range of 32 A. In 3(b) lasing spectra from a laser with L, = 225 J.lm and L2 + L, = 210 J.lm are shown for various current levels. With q = 29 this corresponds to the case of Fig. 2(b) and represents the quasicontinuous wavelength tuning with more than 14 different tunable wavelengths within a range of 37 A. Figure 4 shows lasing spectra from a laser with L, = 225 J.lm and L2 + L1 = 285 J.lm for various current levels. The wavelength tuning range is as large as 90 A. This is the widest range obtained so far with GaAs­GaAIAs laser diodes and is comparable to the 150-A tuning range obtained in Ref. 7 considering the fact that they used a GalnAsP laser with a longer wavelength. For comparison, a tuning range of 2-3 A was obtained by separate pumping an open ended Michelson interferometer (OEMI) laser. K In gen­eral, the wavelength tuning with the SPIEl laser is created by varying both injection currents, but there are regions

I, (mA

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30

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WAVELENGTH (A)

FIG. 4. Lasing spectra ofa laser with L, = 225 flm and L + L, = 285pm at various current levels.

A. Antreasyan and S. Wang 531

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f­:::J "-

10

f- 5 :::J o

I 30 m A CURRENT

FIG. 5. (L-I) characteristic of a singly pumped laser at the cleaved facet of the L, + L, cavity (total cavity length 420/lm).

w here we keep I I unchanged while varying 12 and the tuning rate within those regions was measured to be as much as 1.D-1.5 A/rnA. This rate is much larger than the tuning rate of 100 MHz/rnA obtained by direct modulation of the injec­tion current' and the 0.02 A/V rate obtained by utilizing the electro-optic effect.h Furthermore, in the present scheme us­ing separate pumping, the tuning rate can still be increased by cleaving shorter cavities.

By shorting the two separate electrodes the SPIEl laser can be singly pumped. In this mode of operation a threshold current as low as 30 rnA was obtained as illustrated in Fig. 5 with an external differential quantum efficiency of 40% (20% per facet). This particular measurement was made with a laser of 420-,um total cavity length under pulsed oper­ation. Such low threshold currents permit separately

532 Appl. Phys. Lett., Vol. 43, No.6, 15 September 1983

pumped operation within II + 12 <60 rnA. In addition, the devices are easy to fabricate. Only a standard photolithogra­phic procedure is required to achieve the ohmic contact se­paration.

In conclusion, we have demonstrated a separately pumped semiconductor interferometric laser with a wave­length tuning range as large as 90 A where we distinguish between discrete and quasicontinuous wavelength tuning. Hence, the SPIEl laser can be used as an optical source in wavelength-division-multiplexing systems as well as in FM optical communication systems. Since the carrier and pho­ton dynamics are governed by the rate equations, we expect that the SPIEl laser should be capable of fast response in the gigahertz region." The laser is operable at low current levels comparable to those of straight cavity lasers despite having a curved resonator.

We would like to thank Ismail H. A. Fattah for his advice in the accomplishment of the Ohmic contact separa­tion and Hong K. Choi and K. L. Chen for many helpful discussions. This work was supported by the U.S. Army Re­search Office Contract DAAG29-80-K-OOll, the National Science Foundation ECS-8024307, and the Air Force Office of Scientific Research (AFSC) United Air Force Contract F49620-79-C-0 178.

's. Wang, H. K. Choi, and I. H. A. Fattah, IEEE J. Quantum Electron. QE-18,61O(1982).

'H. K. Choi and S. Wang, Appl. Phys. Lett. 40, 571 (1982). 'I. H. A. Fattah and S. Wang, Appl. Phys. Lett. 41, 112 (1982). ·A. Antreasyan and S. Wang, Appl. Phys. Lett. 42, 562 (1983). 'So Kobayashi, Y. Yamamoto, M. Ito, and T. Kimura, IEEE 1. Quantum Electron. QE-18, 582 (1982).

oF. K. Reinhart and R. A. Logan, Appl. Phys. Lett. 27, 532 (1975). 'W. T. Tsang, N. A. Olsson, and R. A. Logan, Appl. Phys. Lett. 42, 650 (1983).

xI. H. A. Fattah and S. Wang, paper 29BI-3 to be presented at the Fourth International Conference on Integrated Optics and Optical Fiber Com­munication, Tokyo, Japan,1une 1983.

"H. Kressel and 1. K. Butler, Semiconductor Lasers and Heterojunctions LEDs (Academic, New York, 1977), Chap. 17, pp. 575-579.

A. Antreasyan and S. Wang 532

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