We report on a 1550-nm matched filter based on a pair of fiber Bragg gratings (FBGs) that is actively stabilized
over temperature. The filter is constructed of a cascaded pair of athermally-packaged FBGs. The tandem FBG
pair produces an aggregate 3-dB bandwidth of 3.9-GHz that is closely matched to a return-to-zero, 2.880-GHz
differential-phase-shift-keyed optical waveform.
The FBGs comprising the filter are controlled in wavelength using a custom-designed, pulse-width modulation
(PWM) heater controller. The controllers allow tuning of the FBGs over temperature to compensate and cancel
out native temperature dependence of the athermal FBG (AFBG) package. Two heaters are bonded to each
FBG device, one on each end. One heater is a static offset that biases the FBG wavelength positively. The second
heater is a PWM controller that actively moves the FBG wavelength negatively. A temperature sensor measures
the FBGs' temperature, and a feed-forward control loop adjusts the PWM signal to hold the wavelength within
a desired range.
This stabilization technique reduces the device's native temperature dependence from approximately 0.65
pm/°C to 0.06 pm/°C, improving the temperature stability by tenfold, while retaining some control for poten-
tial long-term drifts. The technique demonstrates that the FBGs can be held to ±1.5 pm (±188 MHz) of the
target wavelength over a 0 to +50°C temperature range. The temperature-stabilized FBGs are integrated into
a low-noise, optical pre-amplifier that operates over a wide temperature range for a laser communication system.
We report a single-polarization, optical low-noise pre-amplfier (SP-OLNA) that enhances the receiver sensitivity of heavily-coded 1.55-μm optical communication links. At channel bit-error ratios of approximately 10%, the erbium-doped SP-OLNA provides an approximately 1.0-dB receiver sensitivity enhancement over a conventional two-polarization pre-amplfier.
The SP-OLNA includes three gain stages, each followed by narrow-band athermal fiber Bragg gratings. This cascaded fiter is matched to a return-to-zero, 2.88-Gb/s, variable burst-mode, differential phase shift keying (DPSK) waveform. The SP-OLNA enhancement of approximately 1.0 dB is demonstrated over a range of data rates, from the full 2.88-Gb/s (non-burst) data rate, down to a 1/40th burst rate (72 Mb/s).
The SP-OLNA'sfirst stage of ampli_cation is a single-polarization gain block constructed from polarization-maintaining (PM) fiber components, PM erbium gain fiber, and a PM integrated pump coupler and polarizer. This first stage sets the SP-OLNA's noise figure, measured at 3.4 dB. Two subsequent non-PM gain stages allow the SP-OLNA to provide an overall gain of 78 dB to drive a DPSK demodulator receiver. This receiver is comprised of a delay-line interferometer and balanced photo-receiver. The SP-OLNA is packaged into a compact, 5"x7"x1.6" volume, which includes an electronic digital interface to control and monitor pump lasers, optical switches, and power monitors.
Mobile free-space laser communication systems must reconcile the requirements of low size, weight, and power with the ability to both survive and operate in harsh thermal and mechanical environments. In order to minimize the aperture size and amplifier power requirements of such systems, communication links must exhibit performance near theoretical limits. Such performance requires laser transmitters and receiver filters and interferometers to maintain frequency accuracy to within a couple hundred MHz of the design frequency. We demonstrate an approach to achieving high frequency stability over wide temperature ranges by using conventional DFB lasers, tuned with TEC and current settings, referenced to an HCN molecular frequency standard. A HCN cell absorption line is scanned across the TEC set-point to adjust the DFB laser frequency. Once the center of the line is determined, the TEC set-point is offset as required to obtain frequency agility. To obtain large frequency offsets from an HCN absorption line, as well as continuous laser source operation, a second laser is offset from the reference laser and the resulting beat tone is detected in a photoreceiver and set to the desired offset using a digital frequency-locked loop. Using this arrangement we have demonstrated frequency accuracy and stability of better than 8 MHz RMS over an operational temperature range of 0ºC to 50º C, with operation within minutes following 8 hour soaks at -40º C and 70º C.