We present an experimental study on the suppression of self pulsing in a single stage pulsed master oscillator power amplifier (MOPA), using pump modulation. Many applications require pulsed lasers operating at low repetition rate. Leaving the pump on for long duration without a seed pulse leads to accumulation of amplified spontaneous emission (ASE) and generation of a self-pulse that can damage the laser. For this experiment, the amplifier was fed with peak seed power of 500 mW having a pulse width and repetition rate of 100 ns and 10 kHz respectively. To study the effect of pump modulation on self-pulsing, we carried out the experiment with both a continuous pump and with a pulsed pump. The output was characterized using a constant fractional discriminator (CFD) that generated TTL pulses, in turn fed to a frequency counter. In case of a continuous pump of 2.4 W, with a low repetition rate of 10 kHz for the seed, we noticed self-pulsing of the MOPA. We obtained a maximum amplified signal output peak power of 162 W, or a pulse energy of 7.91 μJ. A further increase in pump power could lead to permanent damage to the system. In case of pulsed pump, the generated pump pulses were synchronized with the seed laser and had a 50 % duty cycle. The resulting output was again characterized with a CFD and counter. We were able to increase the pump power to 6.5 W with an output peak power of 160 W and pulse energy of 9.37 μJ without any sign of self-pulsing. Thus, using a simple method of pump modulation, we were able to reduce the accumulation of ASE and suppressed the phenomenon of self-pulsing at low repetition rates of the seed laser.
The advanced photonics laboratory is a biannual course offered to graduate students, after they have taken a few basic courses in optics and photonics. The course contents include experiments involving different areas in optics and photonics. Learning outcome includes theoretical formulation in optical phenomena and adoption of its instruments. Students are encouraged to explore experimental parameters, observe the effect on measured quantities, correlate them to theoretical predictions and finally explain any discrepancies. The methodology encourages students to question both theoretical models and experimental techniques.
We describes the design, development and testing of a cost effective and miniaturized version of a flow analyzer. It is designed to detect fluorescence labeled immunocytes in human blood sample. Availing of advancements in micro fluidics and nanolithographic technique, we fabricated a PDMS based device with integrated micro channels for accommodating the optical fibers. The lensed fibers serves as the waveguides for illumination and collection of laser and scattered signals respectively. As a cell crosses the interrogation point the forward scatter, side scatter and fluorescence are detected. Photomultiplier tubes used in conventional flow cytometers have been replaced here with APDs (avalanche photo detectors) and supplemented with digital signal processing. The prototype was tested with different sized polymer beads as well as the live cells.
This paper presents a design procedure for a fibre interferometer, the optical system and its associated electronic
control. Analog and digital circuits were optimized to achieve a low cost compact system. The lock-in amplifier
required for phase control was designed using a FPGA. The errors in an interferometric measurement were
studied in detail and its results were used to estimate the capabilities of the interferometer. These matched
our observed resolution of 40 nm. A stabilization technique of controlling the path length difference between
the arms of interferometer nullifies any phase errors. The design and testing of the control circuit are described
in detail. In addition, the FPGA was programmed to carry out phase stepping, as this technique is used to
calculate the desired phase. The interferometer was used to measure samples with step heights in the hundreds
of nanometers, with improvements in accuracy through averaging of data. We verified the successful working of
the instrument by measuring a height of 423 nm for a 420 nm structure.
We describe a frequency-coded scheme to implement the BB84 quantum key distribution protocol using spin
wave (SW)-optical interactions. The interaction of SWs with optical coherent states allows TE↔TM mode
conversion with simultaneous change of frequency and polarization while introducing a phase difference between
the two modes. To implement the BB84 protocol, key bits are encoded as relative phases between the TE and
TM modes. The proposed scheme offers a higher key rate, due to a modulation frequency as high as 25 GHz,
which also relaxes the specifications on the optical filter at the receiver. In addition, SW-optical interactions
yield the added security of truly single-sideband modulation.
One of the challenges in quantum key distribution is to ensure adequate security against eavesdropping. The
B92 protocol achieves this through the transmission of a strong and weak pulse, with key bits coded onto the
weak pulse and channel monitoring done using the bit error rate of the strong pulse. However, the protocol
also assumes synchronization between transmitter and receiver using a shared or transmitted clock signal. We
propose a scheme that uses a mode locked laser for the strong pulse train, and a nonlinear four-wave mixing
processes to generate the weak pulse train. The detector relies on dispersion effects in the transmission channel to
delay the weak pulse relative to the strong pulse, and then use the strong pulse to gate the detection of the weak
pulse. The scheme is thus self-synchronized and offers the option of additional security using four-wave-mixing
Time resolved photon counting was used to separate the different photon states emitted from a strongly attenuated
laser source. We first describe a method to quantify the efficiency of our gated avalanche photo-detector,
by relying on known Poissonian statistics. The detector was then optimized under different temperature and
bias voltage conditions using the noise equivalent power as a metric. Finally, coherent pulses are sent into a ring
cavity, such that the tapped output from the cavity forms a series of time multiplexed pulses, which then yield
the photon counting statistics. We observed good agreement between theoretical estimates and experimental
observations, to as low as 0.01% probability of detection.
We report an all-fibre interferometric autocorrelation measurement of femtosecond optical pulses in the C-band.
An all-fibre Mach-Zehnder interferometer with piezo-electric fiber stretcher in one arm provides the interference
signal. A highly nonlinear and dispersion shifted fibre with Kerr coefficient 10.5 /W-km generates an idler by the
process of degenerate four wave mixing of the pump and target pulse signals. The idler intensity is proportional
to the interferometric autocorrelation of target pulse. We have measured approximately 100 fs optical pulses at
1552.52nm from an actively mode locked erbium doped fibre laser operating at 280 MHz repetition rate.
An estimate of stiction force, rather than the more commonly reported surface energy, helps design reliable structures.
Stiction is a major cause of failure in surface micromachined structures. We report on the modeling and estimation of the stiction force from simple I-V curves on cantilever beams which can be measured even on packaged devices. We have fabricated oxide anchored cantilever beams of polysilicon by surface micromachining. Current is measured for an applied bias between the beam and the substrate. Pull-in and pull-out voltages are determined as the points of maximum slope calculated by differentiating a cubic spline fit to the measured I-V data. The commercial package CoventorWare was used to develop an empirical model for estimating the pull-out voltage for the cases when there is no stiction and in the presence of stiction. A model is developed for finding the stiction force from the simulated and the experimental pull-out voltages. The method uses only measured values of pull-in and pull-out voltages and the beam length and does not require the value of Young's modulus. We also discuss an independent visual method to estimate the process stiction force from the cantilever beam array that is normally used to estimate the surface adhesive energy. An analytical model is developed to calculate the stiction force from the attachment length of long stuck cantilever beams that are released in the same process.