Dynamic Single Mode Lasers and Integrated Lasers ~ Basic Research for Ultra-high Speed and Long Distance Optical Fiber Communications ~

Transmission loss vs. Wavelength of optical silica fiber

Fig.1 Transmission loss vs. Wavelength of optical silica fiber

Schematic structure of a 1.5μm wavelength GaInAsP/InP BH-BIG-DBR Laser.

Fig.2 Schematic structure of a 1.5μm wavelength GaInAsP/InP BH-BIG-DBR Laser.

Fabrication processes of the BH-BIG-DBR laser employed in this device.

Fig.3 Fabrication processes of the BH-BIG-DBR laser employed in this device.

(a)Scanning Electron Microscope (SEM) view of the cross section of a BIG structure after the second crystal growth, and (b) magnified the joint part.

Fig.4 (a)Scanning Electron Microscope (SEM) view of the cross section of a BIG structure after the second crystal growth, and (b) magnified the joint part.

Typical lasing characteristics of a BH-BIG-DBR laser under continuous wave condition with the active region length of 100μm. (a) Light output from the front facet versus injection current and (b) the lasing spectrum.

Fig.5 Typical lasing characteristics of a BH-BIG-DBR laser under continuous wave condition with the active region length of 100μm. (a) Light output from the front facet versus injection current and (b) the lasing spectrum.

Example of lasing spectrum of a sample with active length of 200μm under sinusoidal current modulation at frequency of 1.55Ghz.

Fig.6 Example of lasing spectrum of a sample with active length of 200μm under sinusoidal current modulation at frequency of 1.55Ghz.

Temperature dependences of the lasing wavelength and the threshold current under continuous wave condition for samples with an active region length of (a) 100μm and 50μm

Fig.7 Temperature dependences of the lasing wavelength and the threshold current under continuous wave condition for samples with an active region length of (a) 100μm and 50μm

    Optical fiber communication is an information-communication infrastructure for modern society. Research and development of optical fiber communication had advanced significantly and commercial fiber systems had been installed by 1980s. By then ultra-high speed and long distance optical fiber communication in the lowest loss 1.5-μm wavelength region, which is shown in Fig.1, had been commercialized for the trunk systems. The light source for those ultra-high speed and long distance systems uses a single mode semiconductor laser, which is sometimes called a dynamic single mode laser operating in a stable single mode. The device discussed here is a kind of dynamic single mode laser.

    Another aspect of this device is related to an integrated laser. As shown in Fig. 2, an integrated laser consists of a monolithically integrated active light emitting part and a passive output waveguide part. Figure 2 shows the proposed bundle-integrated guide distributed reflector laser. An integrated laser can monolithically integrate various optical components along the output waveguide such as wavelength selective filter and/or reflector, wavelength tunable parts, optical detector, and optical amplifier. Thus, integrated lasers can fundamentally improve a laser’s properties. They also feature active photonic integrated circuits (PICs), which monolithically integrate many other photonic circuits for stable, compact, high performance operation with surprisingly low cost production. A wavelength tunable laser that could be electrically controlled by the monolithic phase control portion along the output waveguide was demonstrated in 1983. This kind of laser could be used in wavelength division multiplexing (WDM) systems.

    In the 1970s, researchers were attempting to develop a 1.5-μm semiconductor laser and a 1.5-μm dynamic single mode laser for ultra-high speed, long distance optical communication in the lowest loss optical fiber wavelength region. A 1.5-μm GaInAsP/InP laser was developed that could operate continuously at room temperature in 1979, and a 1.5-μm dynamic single mode laser was developed in 1981.

    The concept of a dynamic single mode laser was suggested in 1972 for use in ultra-high speed and long distance optical communications, which are carried out in a stable single mode under an environment in which the temperature and current vary. In contrast to the conventional Fabry-Perot (FP) semiconductor laser whose substantial spectral width is increased by wavelength hops from one mode to another due to variations in temperature variation and/or injection current. Thus, the transmission bandwidth of the optical fiber is severely restricted by the index dispersion due to the wide spectral width inherently associated with conventional FP lasers. It should be noted that the index dispersion can be compensated for by the structural dispersion of fiber, but the degree of compensation is still limited by the nonlinear mixing of transmission light waves, especially for multi-wavelength WDM systems. Thus, the dynamic single mode laser is an indispensable device for ultra-high speed and long distance optical fiber communications.

    The theoretical advancement of dynamic single mode laser operation discussed here was visualized based on the transverse mode control of semiconductor laser for single mode operation on 1972, short cavity operation on 1973, phase-shifted distributed Bragg reflector (DBR) laser and phase-shifted distributed feed-back (DFB) laser expected with high productivity, and dual cavity operation in 1984. The first integrated laser was fabricated as an integrated twin-guide laser on 1975. The 1.5-μm wavelength dynamic single mode laser was developed as mentioned above. The name of “dynamic single mode laser” was used after 1981. The phase shifted DFB laser was realized in 1984.

    The current 1.5-1.6-μm GaInAsP/InP bundle-integrated-guide (BIG) distributed Bragg reflector (DBR) laser, reported in 1987 at the Transactions of the IECE J., is an integrated laser and a dynamic single mode laser, which is a more advanced version of a dynamic single mode laser based on integrated lasers. This integrated laser consists of a new structure called the (BIG), which is shown in Fig. 2, where the active region for laser action and the output waveguide for integration of other optical devices are monolithically fabricated with higher coupling efficiency. Thus, this device can be fabricated more reproducibly with higher dynamic single mode operation properties. This laser consists of GaAsInP/InP emitting at wavelength from 1.5 to 1.6 μm, the lowest loss wavelength region for silica optical fibers.

    As shown in Fig. 2, DBRs are formed along the output waveguide, which selects a specific wavelength, so as to be a dynamic single mode laser. For the sake of simplicity, this type of laser is also called a DBR laser.

    The BIG laser is a novel laser with higher coupling efficiency between the laser region and the output waveguide region, in which makes it a high performance single mode laser. The bundle integrated guide principle is highly reproducible in fabrication with a coupling efficiency of between 95 and 98% between the laser and output waveguide regions. The fabrication process, which is shown in Fig. 3, starts with (a) the liquid phase epitaxial (LPE) growth of the active region and progresses through (b) chemical etching of the active region around the designed area, (c) holographic lithography for DBR corrugation, (d) the second LPE growth, (e) chemical etching of the mesa structure, and finishes with (f) the third LPE growth for the buried hetero (BH) structure. Finally, the lasers are cleaved separately. Figure 4 shows a scanning electron microscope (SEM) view of the cross-section of a BIG structure after second crystal growth and magnified view of it at join part of interest.

    As shown in Fig. 5, the typical threshold current was 27 mA at a lasing wavelength of 1.51μm and an output power of several mW.

    It should be noted that most of the output optical power is emitted for the main lasing mode, but a very small amount of optical power is for the unwanted sub-mode. The ratio of the power of the main mode to that of the suppressed sub-mode is called the sub-mode suppression ratio (SMSR), which is defined through past research of the dynamic single mode laser to identify the single mode operation. SMSR became a standard JIS technical term in 1987 and a standard international technical term of the IEC in 2003. This reflects the wide use of dynamic single mode lasers.

   The lasing spectrum of the present laser as modulated by 1.55 GHz is shown in Fig. 6. An SMSR of 32 dB was observed, which indicates that the sub-mode was suppressed less than 1/1000 of the lasing power level.

    The temperature characteristics are shown in Fig. 7. The lasing wavelength varied with temperature by 1 Angstrom/oC, where the wavelength variation was due to variation in the refractive index of the materials, and no mode hopping was observed. This result is in contrast with the larger temperature variation for conventional FP lasers of 5 Angstrom/oC. These are excellent properties for a dynamic single mode laser.

    This study was done as basic research for the ultra-high speed and long distance optical fiber communications. The Institute of Electrical, Information and Communication Engineers of Japan presented the 1988 Excellent Paper Award to Yuich Tohmori, Kazuhiro Komori, Shigehisa Arai, and Yasuharu Suematsu of the Tokyo Institue of Technology, for the paper "1.5-1.6-μm GaInAsP/InP Bundle-Integrated-Guide (BIG) Distributed Bragg-Reflector (DBR) Lasers," Y.Tohmori, K.Komori, S.Arai, and Y.Suematsu, Trans. IEICE of Japan, Vol.E70, No.5, pp.494-503 (May 1987).



Publications

[1] Y.Tohmori, Y.Suematsu, H.Tsushima, and S.Arai、Wavelength Tuning of GaInAsP/InP Integrated Laser with Butt-Jointed Built-in Distributed Bragg Reflector、1983、Electron. Lett., Vol.19, No.17,pp.656-657
[2] K.Komori, Y.Tohmori, S.Arai, and Y.Suematsu、Bundle-Integrated-Guide (BIG) DBR Type Dynamic-Single-Mode Laser with Short Active Region、1985、Trans. IECE of Japan, Vol.E68, 11, pp.742-744
[3] Y.Suematsu, M.Yamada, and K.Hayashi、A Multi-Hetero-AlGaAs Laser with Integrated Twin-Guide、1975、Proc. IEEE, Vol.63, No.1, pp.208-209 (Jan. 1975)
[4] M.Yamada, H.Nishizawa, and Y.Suematsu、Mode Selectivity in Integrated Twin-Guide Lasers、1976、Trans. IECE of Japan, Vol.E59, No.7, pp.9-10 (July 1976)
[5] K.Kawanishi, Y.Suematsu, and K.Kishino、GaAs-AlGaAs Integated Twin-Guide Lasers with Distributed Bragg Reflectors、1977、IEEE J. Quantum Electron., Vol.QE-13, No.2, pp.64-65 (Feb. 1977)
[6] K.Kishino, Y.Suematsu, K.Utaka, and H.Kawanishi、Monolithic Integration of Laser and Amplifier/Detector by Twin-Guide Structure、1978、Japan. J. Appl. Phys., Vol.17, No.3, pp.589-590 (Mar. 1978)
[7] Y.Sakakibara, K.Furuya, K.Utaka, and Y.Suematsu、Single Mode Oscillation Under High-Speed Direct Modulation in GaInAsP/InP Integrated Twin-Guide Lasers with Distributed Bragg Reflectors、1980、Electron. Lett., Vol.16, No.12, pp.456-458 (June 1980)
[8] Y.Suematsu、Long-Wavelength Optical Fiber Communication、1983、(Invited) Proc. IEEE, Vol.71, No.6, pp.692-721 (June 1983).
[9] Y.Tohmori, Y.Suematsu, H.Tsushima, and S.Arai、Wavelength Tuning of GaInAsP/InP Integrated Laser with Butt-Jointed Built-in Distributed Bragg Reflector、1983、Electron. Lett., Vol.19, No.17,pp.656-657 (Aug. 1983)
[10] Y.Suematsu, S.Arai, and K.Kishino、Dynamic-Single-Mode Semiconductor Lasers with Distributed Reflector、1983、(Invited) IEEE J. Lightwave Technol., Vol.LT-1, No.1, pp.161-176 (Mar. 1983)
[11] M.Asada and Y.Suematsu、Density-Matrix Theory of Semiconductor Lasers with Relaxation Broadening Model-Gain and Gain-Suppression in Semiconductor Lasers、1985、IEEE J. Quantum Electron., Vol.QE-21, No.5, pp.434-442 (May 1985)
[12] K.Komori, S.Arai, Y.Suematsu, M.Aoki, and I.Arima、Proposal of Distributed Reflector (DR) Structure for High Efficiency Dynamic Single Mode (DSM) Lasers、1988、Trans. IEICE of Japan, Vol.E71, No.4, pp.318-320 (Apr. 1988)
[13] Y. SUEMATSU and S. ARAI、Single-Mode Semiconductor Lasers for Long-Wavelength Optical Fiber Communications and Dynamics of Semiconductor Lasers、2000、(Invited) Journal of Selected Topics on Quantum Electronics, Millennium Issue, Vol.6, No.6, pp.1436-1449 (Nov./Dec. 2000)

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1988
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