Roopa Prakash

Low power generation of equalized broadband CW supercontinua using a novel technique incorporating modulation instability of line broadened pump

Continuous-wave(CW) supercontinuum sources find applications in various domains such as imaging, spectroscopy, test and measurement. They are generated by pumping an optical fiber with a CW laser in the anomalous-dispersion region close to its zero-dispersion wavelength. Modulation instability(MI) sidebands are created, and further broadened and equalized by additional nonlinear processes generating the supercontinuum. This necessitates high optical powers and at lower powers, only MI sidebands can be seen without the formation of the supercontinuum. Obtaining a supercontinuum at low, easily manageable optical powers is attractive for many applications, but current techniques cannot achieve this. In this work, we propose a new mechanism for low power supercontinuum generation utilizing the modified MI gain spectrum for a line-broadened, decorrelated pump. A novel two-stage generation mechanism is demonstrated, where the first stage constituting standard telecom fiber slightly broadens the input pump linewidth. However, this process in the presence of dispersion, acts to de-correlate the different spectral components of the pump signal. When this is sent through highly nonlinear fiber near its zero-dispersion wavelength, the shape of the MI gain spectrum is modified, and this process naturally results in the generation of a broadband, equalized supercontinuum source at much lower powers than possible using conventional single stage spectral broadening. Here, we demonstrate a ~0.5W supercontinuum source pumped using a ~4W Erbium-Ytterbium co-doped fiber laser with a bandwidth spanning from 1300nm to 2000nm. We also demonstrate an interesting behaviour of this technique of relative insensitivity to the pump wavelength vis-a-vis zero-dispersion wavelength of the fiber.

A versatile, C-band spanning, high repetition rate, cascaded four wave mixing based multi wavelength source

Demand for bandwidth in optical communications necessitates the development of scalable transceivers that cater to these needs. For this, in DWDM systems with/without Superchannels, the optical source needs to provide a large number of optical carriers. The conventional method of utilizing separate lasers makes the system bulky and inefficient. A multi-wavelength source which spans the entire C-band with sufficient power is needed to replace individual lasers. In addition, multi-wavelength sources at high repetition rates are necessary in various applications such as spectroscopy, astronomical spectrograph calibration, microwave photonics and arbitrary waveform generation. Here, we demonstrate a novel technique for equalized, multi-wavelength source generation which generates over 160 lines at 25GHz repetition rate, spanning the entire C-band with total power >700mW. A 25GHz Comb with 16 lines is generated around 1550nm starting with two individual lasers using a system of directly driven, cascaded intensity and phase modulators. This is then amplified to >1W using an optimized, Erbium-Ytterbium co-doped fiber amplifier. Subsequently, they are passed through Highly NonLinear Fiber at its zero-dispersion wavelength. Through cascaded Four Wave Mixing, a ten-fold increase in the number of lines is demonstrated. A bandwidth of 4.32 THz (174 lines, SNR>15 dB), covering the entire C-band is generated. Enhanced spectral broadening is enabled by two key aspects - Dual laser input provides the optimal temporal profile for spectral broadening while the comb generation prior to amplification enables greater power scaling by suppression of Brillouin scattering. The multi-wavelength source is extremely agile with tunable center frequency and repetition rate.

Frequency offset locked dual-carrier excitation of phase-modulated electro-optic frequency combs for bandwidth scaling and nonlinear spectral broadening

DWDM with/without super-channel based photonic networks require the use of optical carriers with equalized amplitudes and frequency stabilization of adjacent carriers to realize reliable high bandwidth optical communication systems with high spectral efficiency and long reach. Cascading of electro-optic (EO) modulators is a versatile method for generating tuneable, high repetition rate frequency combs which can be used as sources for the carriers. However, the number of lines produced with this technique is limited by the number of phase modulators. Nonlinear spectral broadening is an attractive option for bandwidth scaling; however, bandwidth scaling of single carrier combs through four wave mixing suffers from unequal comb lines or power limitations due to Brillouin scattering. A simpler technique to increase the number of comb lines would involve using multicarrier excitations for comb generation which would result in a proportional increase in the comb lines. Further, dual-carrier excitation enables an excellent temporal profile for nonlinear spectral broadening. However, since the two carriers have uncorrelated drifts, the resultant frequency combs would be unsuitable for most applications. This issue can be overcome by frequency offset locking the two lasers. Here, we demonstrate frequency offset locking (MHz accuracy) of two diode lasers spaced by 100GHz by using an optical phase locked loop which locks one laser to a RF harmonic of the other. This allows for the generation of frequency comb lines locked to each other even post nonlinear broadening. Using this technique, we demonstrate a 25GHz frequency comb with >90 lines (2THz) in the C-band.

Spectrally resolved, broadband frequency response characterization of photodetectors using continuous-wave supercontinuum sources

A simple and powerful method using continuous wave supercontinuum lasers is demonstrated to perform spectrally resolved, broadband frequency response characterization of photodetectors in the NIR Band. In contrast to existing techniques, this method allows for a simple system to achieve the goal, requiring just a standard continuous wave(CW) high-power fiber laser source and an RF spectrum analyzer. From our recent work, we summarize methods to easily convert any high-power fiber laser into a CW supercontinuum. These sources in the time domain exhibit interesting properties all the way down to the femtosecond time scale. This enables measurement of broadband frequency response of photodetectors while the wide optical spectrum of the supercontinuum can be spectrally filtered to obtain this information in a spectrally resolved fashion. The method involves looking at the RF spectrum of the output of a photodetector under test when incident with the supercontinuum. By using prior knowledge of the RF spectrum of the source, the frequency response can be calculated. We utilize two techniques for calibration of the source spectrum, one using a prior measurement and the other relying on a fitted model. Here, we characterize multiple photodetectors from 150MHz bandwidth to >20GHz bandwidth at multiple bands in the NIR region. We utilize a supercontinuum source spanning over 700nm bandwidth from 1300nm to 2000nm. For spectrally resolved measurement, we utilize multiple wavelength bands such as around 1400nm and 1600nm. Interesting behavior was observed in the frequency response of the photodetectors when comparing broadband spectral excitation versus narrower band excitation.

High power, equalized, continuous-wave supercontinuum generation using cascaded Raman fiber amplifiers

Continuous-wave (CW) super-continua have found applications in various domains such as sensing, spectroscopy, test and measurement and generation of broadly tunable lasers. CW supercontinua are implemented by high power pumping of a fiber in the anomalous dispersion regime, close to its zero-dispersion [1-3]. Pumped by Ytterbium doped fiber lasers, photonic crystal fibers are necessary since conventional silica fibers have zero dispersion only beyond 1310nm [1]. However, this approach has limitations arising from the complexities of using photonic crystal fibers. It is desirable to have an all fiber supercontinuum using standard silica fibers. However, sources at the 1.5micron band, where highly nonlinear fibers with zero dispersion are easily available are limited in power. Further, the power density achieved in such sources is poor in the normal dispersion region (between 1–1.5micron) [2-3]. Here, we demonstrate a novel technique for high power, high efficiency, supercontinua generation using the recently proposed cascaded Raman fiber amplifier architecture [4] for Raman lasers.

High power, Continuous Wave, Supercontinuum Generation using Erbium-Ytterbium Co-doped Fiber Lasers

We demonstrate a 3.2W continuous-wave supercontinuum pumped by an Erbium-Ytterbium fiber-laser. A novel, two-fiber cascade for generation is demonstrated with very interesting results. The continuum spanned from 1300nm to ~1900nm with over 300nm at spectral-density>1mW/nm.