Microstructure-fiber optical parametric amplifier operates in the 1550nm telecom band

Microstructure-fiber optical parametric amplifier operates in the 1550nm telecom band
Microstructure Fibers (MFs) have received a lot of attention owing to the high nonlinearity that results from their very small effective core area. Consequently, all the nonlinear effects that are present in standard optical fibers are expected to be observed in MFs with reduced length and power requirements. A number of MF-based telecommunication applications have already been demonstrated, including Raman amplification, 2R regeneration, time-domain demultiplexing, wavelength conversion, and optical switching.

We report, for the first time to our knowledge, a Microstructure Fiber Optical Parametric Amplifier (MFOPA) operating in the 1550nm region. In our preliminary measurements, we demonstrate more than 15dB gain over approximately 20nm range, achieving a peak net gain of 22.5dB.

We used two cascaded pieces of MFs. One reason is that the two pieces allow us to obtain higher four-wave mixing (FWM) gain at a given pump power. The second reason: the two MFs can provide complementary gain spectra due to their different zero-dispersion wavelengths, and as a result, significantly improve the gain bandwidth. We previously demonstrated this kind of two-stage FOPA by using cascaded dispersion-shifted fibers.

The first piece is 12.5 meters long and consists of a silica core with an average diameter of 2.4 +/- 0.2 microns surrounded by a hexagonal array of approximately 0.8 micron diameter air voids. The zero-dispersion wavelength for this piece of MF is 1544 +/- 5nm and the total loss is measured to be 1.7dB, including connector loss on each end of the fiber. The second piece of the MF is 11.0 meters long and has the same geometric properties as the first piece, but the zero-dispersion wavelength and the loss are 1558 +/- 5 nm and 2.2dB (including connector losses), respectively. The approximate zero-dispersion wavelength of the composite 23m MF is 1555 +/- 5 nm.

The pump source for the MFOPA was derived from a continuous-wave distributed-feedback (CW-DFB) laser having 1557.4nm wavelength. In order to achieve high peak pump power with the use of a moderate power erbium-doped fiber amplifier (EDFA), we chopped the continuous-wave light from the DFB laser. A LiNbO3 Mach-Zehnder intensity modulator was employed for this purpose. The resulting pulses had a width of 100ns and a repetition rate of 250kHz. The duty cycle was 1:37.4, taking into account the extinction ratio of amplitude modulation. The pulses were then amplified by an EDFA to compensate for the loss caused by the intensity and the phase modulators and filtered by a 1nm optical-bandpass filter (OBF) to suppress the amplified spontaneous emission (ASE) at the signal wavelengths.

A second EDFA was then used to boost the peak power of the pulses to allow the signal to see gain through the FWM process. The signal source, which was used to measure the net gain of the MFOPA, was a CW laser that is tunable from 1530 to 1580nm. An 80/20 coupler combined the pump with the signal into the cascaded MFs. The input signal power was around 1mW and the pump peak power exceeded 5W.

After being amplified by a train of pump pulses, the amplified signal, which also took on a pulsed shape, was optically filtered and detected with a photodetector and an optical spectrum analyzer. We measured the gain at the peak of the amplified signal pulses after adjusting the fiber polarization controllers (FPC) in both the pump and the signal paths to allow for optimum FWM. The leakage from the strong pump at the signal wavelength was then subtracted and the net signal gain determined.

In conclusion, we have demonstrated the first, to the best of our knowledge, microstructure-fiber optical parametric amplifier in the 1550nm telecom band. In initial measurements we obtained gains >15dB over approximately 20nm bandwidth, achieving a peak net gain of 22.5 dB.

This article will be presented in full in a paper at the Optical Fiber Communications conference titled "Microstructure-fiber based optical parametric amplifier in the 1550nm telecom band ."