Most of the current large mode area (LMA) fibers are few-moded designs using a large, low numerical aperture (N.A.) core, which promotes mode coupling between core modes and increases bending losses (coupling with claddingmodes), which is undesirable both in terms ofmode area and beamquality. Furthermore, short LMA fiber lengths and small cladding diameters are needed to minimize nonlinear effects and maximize pump absorption respectively in high-power pulsed laser systems. Although gain fiber coiling is a widely used technique to filter-out unwanted modes in LMA fibers, coupling between modes can still occur in component leads and relay fibers. In relay fiber, light coupled into higher-order modes can subsequently be lost in the coiling or continue as higher-order modes, which has the overall effect of reducing the effective transmission of the LP01 mode and degrading the beam quality. However, maximum transmission of the LP01 mode is often required in order to have the best possible beam quality (minimal M2). Launching in an LMA fiber with a mode field adapter (MFA)1 provides an excellent way of ensuring maximum LP01 coupling, but preservation of this mode requires highmodal stability in the output fiber. Small cladding, low N.A. LMA fibers have the disadvantage of being extremely sensitive to external forces in real-life applications, which is unwanted for systems where highly sensitive mode coupling can occur. In this paper, we present a detailed experimental and theoretical analysis of mode coupling sensitivity in LMA fibers as a function of fiber parameters such as N.A., core diameter and cladding diameter. Furthermore, we present the impact of higher N.A. as a solution to increase mode stability in terms of its effect on peak power, effective mode area and coupling efficiency.
We present an all-fiber monolithically integrated fiber laser based on a custom tapered fused bundle pump combiner
with 32 inputs ports connected to a double clad gain fiber. The pump combiner is designed to provide high isolation
between signal and pumps fibers providing intrinsic pump protection. This configuration can generate more than 100W
of continuous wave (CW) laser light using single-chip multimode pumps enabling long term reliability.
Differential phase shift keyed transmission (DPSK) is currently under serious consideration as a deployable datamodulation
format for high-capacity optical communication systems due mainly to its 3 dB OSNR advantage over
intensity modulation. However DPSK OSNR requirements are still 3 dB higher than its coherent counter part, PSK.
Some strategies have been proposed to reduce this penalty through multichip soft detection but the improvement is
limited to 0.3dB at BER 10-3. Better performance is expected from other soft-detection schemes using feedback control
but the implementation is not straight forward. We present here an optical multipath error correction technique for
differentially encoded modulation formats such as differential-phase-shift-keying (DPSK) and differential polarization
shift keying (DPolSK) for fiber-based and free-space communication. This multipath error correction method combines
optical and electronic logic gates. The scheme can easily be implemented using commercially available interferometers
and high speed logic gates and does not require any data overhead therefore does not affect the effective bandwidth of
the transmitted data. It is not merely compatible but also complementary to error correction codes commonly used in
optical transmission systems such as forward-error-correction (FEC). The technique consists of separating the
demodulation at the receiver in multiple paths. Each path consists of a Mach-Zehnder interferometer with an integer bit
delay and a different delay is used in each path. Some basic logical operations follow and the three paths are compared
using a simple majority vote algorithm. Receiver sensitivity is improved by 0.35 dB in simulations and 1.5 dB
experimentally at BER of 10-3.
Differential-phase-shift-keyed optical modulation (DPSK) has generated a lot of attention in fiber optic transmission over the past few years mainly because of its 3dB optical signal-to-noise ratio (OSNR) improvement over standard intensity modulated transmission  offering high receiver sensitivity, high tolerance to major nonlinear effects in high-speed transmissions , and high tolerance to coherent crosstalk .
To demodulate DPSK, a delay-line interferometer is usually employed to provide a one-bit delay such that a bit interferes on the following bit to provide constructive or destructive interference depending on the phase difference . However, with the use of logical pre-coding a multi-bit delay can be used instead of a single-bit delay. It was recently reported that a two- or four-bit delay might be advantageous in allowing polarization interleaving between bits to lessen the detrimental effects of fiber nonlinearities in fiber optic transmission [5-7]. Multi-bit delay was also proposed as a method to correct errors from amplified spontaneous emission (ASE) noise-limited transmission [8-9].
We present here simulation and experimental results on the penalty of using multi-bit delay demodulation for DPSK detection. We present Q-factor degradation as a function of delay and find that the Q penalty scales with 0.5 x delay for integer delays. We also present results of the detrimental effect of spectral filtering from the reduced free-spectral-range (FSR) in a multi-bit delay interferometer. We also find that it exhibits reduced dispersion tolerance. If not taken into account, these important limitations and more stringent tolerance on the frequency offset may reduce the effectiveness of multi-bit delay methods and prohibit practical implementation.
Mach-Zehnder interferometers (MZIs) are used in many optical applications, such as measurement of the coherence length of a laser, thermal dynamic flow, flatness of plane optical plates, thickness of thin films, etc. In this type of interferometer, light passing through a sample region in one direction recombines with a second leg without traversing the sample twice.
In telecommunication, MZIs are used for demodulating differential phase-shift-keyed (DPSK) signals. DSPK has attracted increased attention in fiber optic transmission in recent years because of its 3-dB optical signal to noise ratio improvement over standard intensity modulated transmission, as well as for its high tolerance to nonlinear effects and coherent crosstalk. In a standard fiber MZI, two wideband fiber couplers are spliced together with one arm providing a one-bit delay to convert the phase difference into an intensity modulation. In our alternate type of MZI, the two-mode interferometer, the time delay is obtained through the difference between the propagation constants of two modes instead of through a physical path length difference. We present here a novel single multimode fiber modal interferometer for DPSK demodulation. In this design, a second mode is excited by splicing a standard fiber to a multimode fiber length such that two modes beat together before recombining in a second splice. A numerical analysis and an experimental verification of the multimode fiber parameters to maximize the extinction ratio and minimize the length of the interferometer are presented. We investigate coupling, insertion losses, temperature sensitivity and polarization effects of using modes with and without radial symmetry. The design is extremely low-cost, easily manufactured and is intrinsically less temperature sensitive than standard MZI. Although balanced detection is lost, DPSK may still be advantageous because of its high tolerance to nonlinear effects and coherent crosstalk.
Optical packet switching (OPS) and optical burst switching (OBS) are regarded as next-generation transport technologies that enable more efficient and flexible utilization of the capacity of optical networks by providing sub-wavelength granularity. Optically labelled packet transmission based on orthogonal intensity-modulation/differential-phase-shift-keying (IM/DPSK) modulation format, in which the payload is intensity modulated while the label is carried by DPSK has been proposed and demonstrated. More recently, it was found that using DPSK/IM for payload/label modulation and a balanced receiver for DPSK detection is more advantageous. In these optical label encoding schemes, two optical modulators are required, one for encoding the payload and the other for the label. In this paper, we demonstrate a novel payload and label encoding technique based on a single Mach Zehnder (MZ) modulator. In this scheme, the RF port of the MZ modulator is used to encode a 10G DPSK payload while the bias port is used to impose the label information through an appropriate intensity modulation. Direct detection of the label is achieved with an inexpensive low-speed receiver while the DPSK payload is decoded by using an optical 1-bit delay interferometer before detection by either a single or a balanced detector. Experimental results show superior receiver sensitivity for both the label and the payload, which compares favourably with previous reported schemes and with the advantage of using only a single modulator. Furthermore, we show that label removal and re-insertion can be realized in the RF domain without any polarization dependence.
For the last decade, recirculating loops have been a useful tool in the research and development of long haul transmission links. A loop experiment can emulate the transmission of an optical signal over thousands of kilometers by using a relatively short link of a few hundred kilometers and recirculating the signal several times. Although recirculating loops accurately replicate most physical effects encountered in point-to-point links (loss, noise, chromatic dispersion, nonlinear effects, etc), the statistics of polarization effects (polarization mode dispersion (PMD) and polarization-dependent loss (PDL)) may not be properly emulated. In an optical link, PDL can induce statistical fluctuations of the optical signal-to-noise ratio (OSNR) and consequently of the bit-error-rate (BER). Due to environmental changes, the effects of PDL vary stochastically in time. The periodic nature of fiber loop may artificially produce an unrealistic PDL distribution and the statistical distribution of PDL effect may be significantly different from that in a installed link. We report the analysis and observation of a power oscillation effect caused by PDL due to the periodic nature of the polarization evolution in a recirculating loop. The oscillation is expected to affect the OSNR and consequently the BER as a function of recirculation.