Sharad Bhooplapur

Advanced Ultrafast Technologies Based on Optical Frequency Combs

This paper presents recent results in the development of novel ultrafast technologies based on the generation and application of stabilized optical frequency combs. By using novel active resonant cavity injection locking techniques, filtering, modulation and detection can be performed directly on individual components of the frequency comb enabling new approaches to optical waveform synthesis, waveform detection and matched filtering, with effective signal processing bandwidths in excess of 1 THz.

Dynamic line-by-line pulse shaping with GHz update rate

We introduce a novel scheme for dynamic line-by-line pulse shaping with GHz update rates. Four lines of an optical frequency comb source are used to injection-lock four individual VCSEL, which are subsequently electrically modulated at 0.4 to 1 GHz through current modulation. This concept could be considered a completely new way of pulse shaping as the light is not simply modified, but rather regenerated with the desired properties. We also discuss an important drawback of line-by-line pulse shapers that ultimately limits the modulation speed capability.

Direct Demodulation and Channel Filtering of Phase-Modulated Signals Using an Injection-Locked VCSEL

Detection of phase-modulated optical signals without the use of an externally generated local oscillator is demonstrated using an injection-locked vertical-cavity surface-emitting laser. When it is used in a multichannel system, this technique is also capable of actively filtering out one of the phase-modulated optical carriers. The experimental results of the proposed receiver performance in a three-channel optical system are presented here. A signal-to-noise ratio of 75 dBc/Hz is obtained.

Pulse shapes reconfigured on a pulse-to-pulse time scale by using an array of injection-locked VCSELs

We demonstrate line-by-line pulse shaping of optical comb lines separated by 6.25 GHz. An array of injection-locked VCSELs independently modulate four optical comb lines at frequencies up to 3.125 GHz, updating the pulse shape on the time scale of the pulse period.

Characterization of the Phase and Amplitude Modulation of Injection-Locked VCSELs at 1550 nm Using Coherent Optical Demodulation

Experimental measurements of the optical amplitude and phase modulation from an injection-locked VCSEL at 1550 nm are reported. The use of a coherent optical demodulation technique, using a 90° optical hybrid, balanced photodiodes, and a high-speed oscilloscope, has enabled the complete modulation response to be recorded as a function of time. The VCSELs response to current modulation was examined under various experimental parameters: over a wide frequency range, from kilohertz to gigahertz, for small and large current modulation and for different bias currents. Low-frequency (<;1 MHz) current modulation leads to temperature modulation, where the injection-locked VCSEL acts predominantly as a phase modulator, with a maximum depth of phase modulation, β, of 0.34 π rad. In the high-frequency (>100 MHz) electronic modulation regime, the VCSEL produces significant amplitude and phase modulation, with a maximum depth of amplitude modulation, m, of 35% and β = 0.47 π rad. The VCSELs thermal response is much more efficient (requires less RF power) in producing phase modulation than its electronic response. The detailed experimental data on the optical phase modulation response of an injection-locked VCSEL are reported for the first time here and are useful for its many applications.

Linear Technique for Discrimination of Optically Coded Waveforms Using Optical Frequency Combs

We have demonstrated a novel coherent optical detection architecture that serves as a matched filter and successfully discriminates between various encoded optical waveforms with high confidence. The waveforms are produced by encoding the spectral phase of an optical frequency comb with codes from a Walsh-Hadamard set. The encoded optical waveforms are decoded with a coherent detection technique implemented using an optical frequency comb as a set of local oscillators, an interferometer, and a pair of differential balanced photodetectors. The use of only linear optical devices in the receiver results in a higher sensitivity compared to the use of nonlinear optical thresholding and gating techniques. Decoding is demonstrated at power levels of ~ 25 μW, which are much lower than previously reported. Possible applications of the receiver architecture include laser radar and optical communications in an optical code-division multiple access network.

Injection-locked VCSEL arrays for line-by-line pulse-shaping with update times of less than 1 ns

Dynamic and static optical pulse shaping with line-by-line control is shown, using a 12-channel injection-locked VCSEL linear array. With channel modulation frequencies up to 3.125 GHz, pulse shape changes within 1 ns are conclusively demonstrated.

Coherent optical measurement of the modulation dynamics of injection-locked VCSELs

We independently measure the optical phase and amplitude modulation characteristics of an injection-locked VCSEL for the first time at GHz rates, using a coherent optical demodulation scheme, giving us insight into two different phase modulation regimes.

Pattern recognition of electronic bit-sequences using a semiconductor mode-locked laser and spatial light modulators

A novel scheme for recognition of electronic bit-sequences is demonstrated. Two electronic bit-sequences that are to be compared are each mapped to a unique code from a set of Walsh-Hadamard codes. The codes are then encoded in parallel on the spectral phase of the frequency comb lines from a frequency-stabilized mode-locked semiconductor laser. Phase encoding is achieved by using two independent spatial light modulators based on liquid crystal arrays. Encoded pulses are compared using interferometric pulse detection and differential balanced photodetection. Orthogonal codes eight bits long are compared, and matched codes are successfully distinguished from mismatched codes with very low error rates, of around 10-18. This technique has potential for high-speed, high accuracy recognition of bit-sequences, with applications in keyword searches and internet protocol packet routing.

Line-by-Line Pulse-Shaping at GHz Modulation Frequencies With an Injection-Locked VCSEL Array

A 12-channel linear vertical-cavity surface-emitting laser (VCSEL) array emitting at ~1540 nm is used as a modulator array for rapid-update ultrashort pulse shaping. Each VCSEL is injection locked with an individual comb line from a 12.5-GHz optical frequency comb. Pulse shaping is achieved by modulating the current to each VCSEL. Two regimes are demonstrated: 1) static pulse shapes, where the dc bias to the array is set to generate some of the canonical pulse shapes and 2) dynamic pulse shapes, where the currents to the array are modulated at frequencies up to 3.125 GHz, and the resultant pulse shapes change on a sub-ns timescale.