Mohammed N. Islam

Raman amplifiers for telecommunications

Raman amplifiers are being deployed in almost every new long-haul and ultralong-haul fiber-optic transmission systems, making them one of the first widely commercialized nonlinear optical devices in telecommunications. This paper reviews some of the technical reasons behind the wide-spread acceptance of Raman technology. Distributed Raman amplifiers improve the noise figure and reduce the nonlinear penalty of fiber systems, allowing for longer amplifier spans, higher bit rates, closer channel spacing, and operation near the zero-dispersion wavelength. Lumped or discrete Raman amplifiers are primarily used to increase the capacity of fiber-optic networks, opening up new wavelength windows for wavelength-division multiplexing such as the 1300 nm, 1400 nm, or short-wavelength S-band. As an example, using a cascade of S-band lumped amplifiers, a 20-channel, OC-192 system is shown that propagates over 867 km of standard, single-mode fiber. Raman amplifiers provide a simple single platform for long-haul and ultralong-haul amplifier needs and, therefore, should see a wide range of deployment in the next few years.

Mid-infrared supercontinuum generation to 4.5 μm in ZBLAN fluoride fibers by nanosecond diode pumping

A mid-infrared supercontinuum (SC) is generated in ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF...) fluoride fibers from amplified nanosecond laser diode pulses with a continuous spectrum from approximately 0.8 microm to beyond 4.5 microm. The SC has an average power of approximately 23 mW, a pump-to-SC power conversion efficiency exceeding 50%, and a spectral power density of approximately -20 dBm/nm over a large fraction of the spectrum. The SC generation is initiated by the breakup of nanosecond laser diode pulses into femtosecond pulses through modulation instability, and the spectrum is then broadened primarily through fiber nonlinearities in approximately 2-7 m lengths of ZBLAN fiber. The SC long-wavelength edge is consistent with the intrinsic ZBLAN material absorption.

Supercontinuum generation from ~1.9 to 4.5 μmin ZBLAN fiber with high average power generation beyond 3.8 μm using a thulium-doped fiber amplifier

A mid-IR supercontinuum (SC) fiber laser based on a thulium-doped fiber amplifier (TDFA) is demonstrated. A continuous spectrum extending from ∼1.9 to 4.5 μm is generated with ∼0.7 W time-average power in wavelengths beyond 3.8 μm. The laser outputs a total average power of up to ∼2.6 W from ∼8.5 m length of ZrF4─BaF2─LaF3─AlF3─NaF (ZBLAN) fiber, with an optical conversion efficiency of ∼9% from the TDFA pump to the mid-IR SC. Optimal efficiency in generating wavelengths beyond 3.8 μm is achieved by reducing the losses in the TDFA stage and optimizing the ZBLAN fiber length. We demonstrate a novel (to our knowledge) approach of generating modulation instability-initiated SC starting from 1.55 μm by splitting the spectral shifting process into two steps. In the first step, amplified approximately nanosecond-long 1.55 μm laser diode pulses with ∼2.5 kW peak power generate a SC extending beyond 2.1 μm in ∼25 m length of standard single-mode fiber (SMF). The ∼2 μm wavelength components at the standard SMF output are amplified in a TDFA and coupled into ZBLAN fiber leading to mid-IR SC generation. Up to ∼270 nm SC long wavelength edge extension and ∼2.5× higher optical conversion efficiency to wavelengths beyond 3.8 μm are achieved by switching an Er:Yb-based power amplifier stage with a TDFA. The laser also demonstrates scalability in the average output power with respect to the pulse repetition rate and the amplifier pump power. Numerical simulations are performed by solving the generalized nonlinear Schrodinger equation, which show the long wavelength edge of the SC to be limited by the loss in ZBLAN.

10.5 W Time-Averaged Power Mid-IR Supercontinuum Generation Extending Beyond 4

A novel, all-fiber-integrated supercontinuum (SC) laser is demonstrated and provides up to 10.5 W time-averaged power with a continuous spectrum from ~0.8 to 4 mum. The SC is generated in a combination of standard single-mode fibers and ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fluoride fibers pumped by a laser-diode-based cladding-pumped fiber amplifier system. The output SC pulse pattern can be modulated by directly modulating the seed laser diode. Near-diffraction-limited beam qualities are maintained over the entire SC spectrum. The SC average power is also linearly scalable by varying the input pump power and pulse repetition rate. We further investigate the theoretical limitations on the achievable average power handling and spectral width for the SC generation in ZBLAN fibers. Based on the thermal modeling, the standard ZBLAN fiber can handle a time-averaged power up to ~15 W, which can be further scaled up to ~40 W with a proper thermal coating applied onto the ZBLAN fiber. The SC long-wavelength edge is limited by the nonlinear wavelength generation processes, fiber bend-induced loss, and glass material loss. By using a ZBLAN fiber with a 0.3 numerical aperture, the SC spectrum could extend out to ~4.5 mum, which is then limited by the material loss.

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By using a loop configuration formed by a polarization beam splitter, we experimentally demonstrate that the existing wavelength-division multiplexing (WDM) sources in C-band can be wavelength converted to the S-band with low polarization sensitivity and low crosstalk. Using a fiber parametric amplifier as a band converter, we achieve experimentally 4.7-dB conversion efficiency over 30-nm conversion bandwidth in 315 m of fiber. Compared to the conventional straight fiber wavelength conversion scheme, a more than 2-dB improvement in polarization sensitivity is measured. In addition to the polarization insensitivity, channel crosstalk is measured to be <-27 dB in 315 m of high nonlinearity fiber. In a detailed experimental study, the pattern of crosstalk in longer fiber lengths and the coupling between the polarization sensitivity and crosstalk are measured. For example, with a 430-m fiber length, we measure the degradation in polarization sensitivity to be /spl sim/4 dB for 12-dB increased signal power. The experimental results are also confirmed by theoretical calculations. Moreover, in a 32 channels systems simulation, the signal-to-noise ratio (SNR) of the converted signals after 800-km propagation is calculated to be only 0.8-dB degraded compared to using laser diodes with the same initial SNR values. Furthermore, we calculate the effect of pump noise and show that the relative intensity noise of the pump is transferred to the converted signals with an additional 8-dB/Hz degradation.

m With Direct Pulse Pattern Modulation

Mid-infrared supercontinuum (SC) extending to ~4.0 mum is generated with 1.3 W time-averaged power, the highest power to our knowledge, in ZBLAN (ZrF(4)-BaF(2)-LaF(3)-AlF(3)-NaF...) fluoride fiber by using cladding-pumped fiber amplifiers and modulated laser diode pulses. We demonstrate the scalability of the SC average power by varying the pump pulse repetition rate while maintaining the similar peak power. Simulation results obtained by solving the generalized nonlinear Schrödinger equation show that the long wavelength edge of the SC is primarily determined by the peak pump power in the ZBLAN fiber.

Fiber parametric amplifiers for wavelength band conversion

In this letter, we demonstrate a photonic analog-to-digital converter with time stretch (TS) preprocessor that has a sampling rate of 130 GSa/s. The system has a signal-to-noise ratio (SNR) exceeding seven effective number of bits over a 1-GHz bandwidth at 18 GHz. We present an analytical model of the SNR in the TS preprocessor which shows that over the specified bandwidth, the SNR is limited by the amplified spontaneous emission beat noise.

Power scalable mid-infrared supercontinuum generation in ZBLAN fluoride fibers with up to 1.3 watts time-averaged power

We demonstrate a high-power, multi-wavelength, short pulse source at 10 Gb/s based on spectral slicing of supercontinuum (SC) generated in short fibers. We show that short fiber SC can be used for dense wavelength division multiplexing applications because of its >7.9 dBm/nm power spectral density, 140 nm spectral bandwidth, and /spl plusmn/0.5 dB spectral uniformity over 40 mn. Pulse carving up to 60 nm away from the pump wavelength and CW generation by longitudinal mode carving indicates that the coherence of the SC is maintained. By using high nonlinearity fibers, the spectral bandwidth is increased to 250 nm, which can accommodate >600 wavelength channels with 50 GHz channel spacing and >6 Tb/s aggregate data rate. We also calculate the coherence degradation due to amplification of incoherent energy during the SC generation. Theoretical results show that the SC generation in short fibers has 13 dB higher signal-to-noise ratio (SNR) compared to the SC generated in long fiber.

130-GSa/s photonic analog-to-digital converter with time stretch preprocessor

Using an erbium-doped fiber laser (EDFL) passively mode locked by a semiconductor saturable absorber, we generate 5.5-ps pulses of a 2.3-nJ/pulse, which are more than three times higher in energy than for other reported EDFLs. We show that, by introduction of a linear loss element within the cavity, multiple pulsing behavior at high pump powers can be suppressed. We also determine the saturable-absorber characteristics-absorbance versus wavelength near band gap-that are necessary to produce short mode-locked pulses.

10 Gb/s multiple wavelength, coherent short pulse source based on spectral carving of supercontinuum generated in fibers

Packet-drop function for a time-division multiplexing network using 100-Gb/s, 8-b words is experimentally demonstrated by integrating all-optical header processing and payload demultiplexing with electrooptic packet routing. The header processor consists of two levels of all-optical logic gates based on low birefringent nonlinear optical loop mirrors (NOLMs), and the payload demultiplexer is a two-wavelength NOLM. Both devices are driven by synchronized lasers with timing jitter under 1 ps. The contrast ratios for both header processor and demultiplexer are 10:1 and that of the packet router is 17 dB. The switching energies for header processing and payload reading are 10 and 1 pJ/pulse, respectively.