OSA/OFC/NFOEC 2011
JThA18.pdf JThA18.pdf
Compact Tunable Delay Interferometer using Movable Prism for DPSK Optical Performance Monitor Takashi Mori1 , Masatoshi Kagawa2 , Koji Inafune2 , Yoshihiro Kanda2 , Akihito Otani1 and Hitoshi Murai2 1 Anritsu
Corporation, 5–1–1 Onna, Atsugi-shi, Kanagawa 243–8555, Japan. Tel: +81–46–296–6542, Fax: +81–46–225–8302, E-mail:
[email protected] 2 Oki Electric Industry Co., Ltd., 1–16–8 Chuo, Warabi-shi, Saitama 335–8510, Japan.
Abstract: A tunable delay interferometer for a multi-bit-rate DPSK optical performance monitor is described. Quality monitoring of various bit rate signals using all-optical sampling is demonstrated and agrees with the bit error rate. c 2011 Optical Society of America
OCIS codes: (060.5060) Phase modulation; (120.3180) Interferometry; (060.2330) Fiber optics communications
1. Introduction Differential phase shift keying (DPSK) modulation has many advantages, such as a 3-dB reduction in required optical signal-to-noise ratio (OSNR) using balanced detection [1]. A delay interferometer (DI) is used commonly for direct detection of DPSK signals without expensive coherent detection. We have already proposed an optical performance monitor for on-off keying (OOK) signals based on all-optical sampling using an electroabsorption modulator (EAM) [2], and have monitored various-bit-rate OOK signals using a variable-width sampling gate [3]. A DI with tunable free spectral range (FSR) is required for the optical performance monitor for various-bit-rate DPSK signals. The previous DI used precision actuators with nanometer resolution to tune both the FSR and optical phase [4, 5]. However, in general, these actuators are comparatively large. This paper describes the development of a compact, tunable DI with new movable prisms structure using small actuators with micrometer resolution. Quality monitoring of DPSK signals at various bit rates is described using the DI with tunable FSR and all-optical sampling with variable gate width. 2. Setup and characteristics of tunable DI The developed DI is shown in Fig. 1. It has a Michelson interferometer with two output ports. To keep the DI as small as possible, actuators 1 and 2 using lead zirconate titanate (PZT) bi/mono-morph bending deflective vibrations are used to control the carrier phase and FSR. These compact actuators have a comparatively wide dynamic range, as well as an optical positional measurement system with 1-µm resolution. To control the carrier phase, a 1 ◦ wedge prism is pushed and pulled vertically in the optical path by actuator 1 and the resolution is converted to about 8 nm, corresponding to 2◦ of carrier phase. The carrier phase of the DQPSK signal can be adjusted by this phase resolution. In addition, actuator 2 in the developed DI provides wide-delay dynamic range. Actuator 2 has three 30 ◦ wedge prisms and the delay tunability is from 50 to 100 ps, corresponding to a FSR from 20 to 10 GHz. The positions of these actuators are calculated by a CPU to control the FSR and interference phase. It is well known that the phase difference between two orthogonal polarizations impairs the demodulated signal [4], so a wave plate is positioned in one arm of the DI to reduce the impairment. Incident angles to prisms are generally designed to minimize the polarization dependent loss (PDL) caused by mismatch of the anti-reflection (AR) coating. Therefore, light travels along the normal line of the halved-plate of the wedge, and the incident angles are 23◦ assuming a refractive index of 1.5. The reflected TE and TM waves are small
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70 mm
17 mm
(a)
115 mm
(b)
Fig. 1. Setup of tunable delay interferometer. (a) Schematic, (b) internal photograph.
OSA/OFC/NFOEC 2011
JThA18.pdf JThA18.pdf
º
º
(a)
(b)
23º
Table 1 Typical DI specifications
15º
º
Port1 Port2 1.5 2.3
