OPTOELEOTRONICS LETTERS Vol.2 No.5,15 Sep. 2006 Fabrication and characterization of 1 550 nm polarization-in- sensitive semiconductor optical amplifiers HU Yong-hong*, HUANG Yong-zhen, YU Li-juan, CHEN Qin, TAN Man-qing, and MA Xiao-yu State Key Laboratory on Integrated Optoelectronics,Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China (Received 17 May 2006) A 1 550 nm polarization-insensitive semiconductor optical amplifier (SOA) was fabricated with InGaAs tensile-strained bulk active region. Beam propagation method and planar wave expansion method are used to calculate the mode field profile and the mode reflectivity. For the SOA with a buried waveguide deviated 7~ from the normal direction of cleaved mirrors, the thickness tolerance of the mirror is 3 % for keeping the reflectivity of TE mode and TM mode less than 10-4 simultaneously. For a SOA with a cav- ity length of 800 ~am, the polarization sensitivity of amplified spontaneous emission spectra is less than 0.5 dB at an injection current of 250 mA,the corresponding fiber-to-fiber gain is 11.9 dB at 1 550 nm with a 3 dB bandwidth of 63 nm, and the saturation output power is 5.6 dBm. The noise figure shows 8.8 and 7.8 dB at 1550 and 1 570 nm,respectively. For a packaged SOA with a cavity length of 1 000 tam,the fiber-to-fiber gain is 15 dB at an injection current of 190 mA. CLC number:TN365 Document code=A Article ID.1673-1905(2006)05-0351-03 Semiconductor Optical Amplifiers (SOAs) can be used as in-line amplifier, preamplifier, optical switch, and wavelength converter in future opticM systemsE13j. Po- larization-independent gain, high output power and low gain ripple are desirable features for most applications. Polarization-independent SOA is mainly based on tensile strained active layerE~ r~. Moreover, square cross section active layer structure can realize polarization independ- ence. To achieve high output power and low gain ripple, low residual reflectivity is required. Antireflection (AR) coating on the both facets of a SOA is a key technique for realizing traveling wave SOA. An angled facet struc- ture or window facet structure can reduce facet reflectiv- ity greatly and can release the strict requirement on AR coating process [~-~<. In this paper,we report the fabrication of a polariza- tion-independent SOA with an active region of InGaAs tensile-strained bulk material. We analyze the polariza- tion sensitivity relationship between amplified spontane- ous emission (ASE) spectra and gain spectra, design AR coating mirror structure for the SOA, and present the measured gain characteristics of the fabricated SOA. Actually,a traveling wave SOA inevitably has a very iow residuaI ref[ectivity R. Considering the refiectivity R at two sides of the SOA, we can express the ASE output spectrum as I = S[-exp(gL) -- 1](1 --R)(1 4- RG,) (1) g[1 -- 2RG, cos(4rcnfL ) 4- R 2 G{ ] C * E-mail: yzhuang@ semi. ac. cn where G,=exp(gL) is the single-pass amplifier gain,g is the net mode gain,L is the cavity length,n is mode refractive index,c is light speed in vacuum,s is the spon- taneous emission spectrum coupled into the waveguide mode,and f is the light frequency. The gain spectrum of the SOA can be expressed as O = (1 -- R)2G~ (2) 1 -- 2RG~cos[ 4nnLf~ + R 2 G~ C When the residual reflectivity R is zero, the gain G of the SOA is equal to the single-pass amplifier gain G,. The residual reflectivity causes gain ripple and limits the a- vailable gain of the SOA. From Eqs. (1) and (2) we can get the following relation between the ASE spectra and gain spectra= I = S(1 +RG,)(G, --1)G (3) g(1 --R)G, Because S/g is a function of energy and is independent of the polarizationE~j , we can obtain the ratio of polari- zation sensitivity between ASh spectra and gain spectra from Eq. (3) as 9 FE/F M (I• GTE~rGTE--1)G~ M _ _ T TE s " \ s OrE / O TM ~ (1 rM TM +R~G )(Gi -1)GY (4) Here/re and/rM ,OwE and @s TM ,and RTE and RTM are the ASE spectra, the single pass gain, and the residual mode reflectivities of TE and TM modes, respectively. In Fig. 1, the polarization sensitivity difference between ASE spectra and gain spectra is plotted as a function of Cr TE