High Relative Intensity Noise Research Articles

High-Speed Balanced-Detection Visible-Light Optical Coherence Tomography in the Human Retina Using Subpixel Spectrometer Calibration.

Increases in speed and sensitivity enabled rapid clinical adoption of optical coherence tomography (OCT) in ophthalmology. Recently, visible-light OCT (vis-OCT) achieved ultrahigh axial resolution, improved tissue contrast, and provided new functional imaging capabilities, demonstrating the potential to improve clinical care further. However, limited speed and sensitivity caused by the high relative intensity noise (RIN) in supercontinuum lasers impeded the clinical adoption of vis-OCT. To overcome these limitations, we developed balanced-detection vis-OCT (BD-vis-OCT), which uses two calibrated spectrometers to cancel RIN and other noises. We analyzed the RIN to achieve robust subpixel calibration between the two spectrometers and showed that BD-vis-OCT reduced the A-line noise floor by up to 20.5 dB. Metrics comparing signal-to-noise-ratios showed similar image qualities across multiple reference arm powers, a hallmark of operation near the shot-noise limit. We imaged healthy human retinas at an A-line rate of 125 kHz and a field-of-view up to 10 mm ×4 mm. We found that BD-vis-OCT revealed retinal anatomical features previously obscured by the noise floor.

Adaptive noise canceling for transient absorption microscopy.

.Significance: Ultrafast fiber lasers are an attractive alternative to bulk lasers for nonlinear optical microscopy for their compactness and low cost. The high relative intensity noise (RIN) of these lasers poses a challenge for pump-probe measurements such as transient absorption and stimulated Raman scattering, along with modalities that provide label-free contrast from the vibrational and electronic structure of molecules.Aim: Digital adaptive filtering was applied to determine the applicability for canceling laser RIN in a transient absorption microscope with an ultrafast fiber laser source.Approach: Digitized signals from the transmitted probe and reference photodetectors were fed to an adaptive filter in MATLAB, running in a noise canceling configuration. This result was then fed to a software lock-in algorithm to demodulate the pump-probe signal. Images were built up one line scan at a time with a 3.5-kHz resonant scanner, with averaging. The imaging target was , which exhibits nondegenerate two-photon absorption at the pump/probe wavelengths used (530-nm pump and 490-nm probe).Results: Without adaptive noise cancellation, the lock-in output primarily passes the laser RIN within its detection bandwidth, resulting in images that closely follow the linear transmissivity and lack sensitivity to pump-probe time delay. With adaptive noise cancellation in front of the lock-in, the RIN rejection is enough to restore the z-sectioning and sensitivity to pump-probe delay, as expected for transient absorption. Results were limited primarily by noise from the photodetector and analog-to-digital converter.Conclusions: Digital adaptive noise cancellation, even when limited by electronics noise, can recover pump-probe signals by removal of laser RIN, under conditions where averaging alone fails.

On the Phase Noise Enhancement of a Continuous Wave in Saturated SOA Used for RIN Reduction

We report on the spectral characteristics of the phase noise enhancement of continuous-wave (CW) optical signals passing through a semiconductor optical amplifier (SOA) operating in the saturation regime and used for relative intensity noise (RIN) reduction. We show that the known effect of phase noise enhancement, attributed to the linewidth enhancement factor of the device, happens only at a limited band of the phase noise spectrum, and the actual measurable linewidth of the output CW signal may not be affected. While this phase noise enhancement is not shown as an increase of spectral linewidth, it can still affect system performance when coherent detection is used, especially in applications with relatively low symbol rates. Numerical simulations and experimental results are used to support the observation. A single spectral line from a quantum-dot mode-locked laser is used as the light source, which is known to have relatively high RIN (> −120 dB/Hz in the low frequency region). Experimental transmission of 16-QAM modulation with coherent detection has been performed at 5 GBd to assess the implication on system performance.

Noise and spectral stability of deep-UV gas-filled fiber-based supercontinuum sources driven by ultrafast mid-IR pulses

Deep-UV (DUV) supercontinuum (SC) sources based on gas-filled hollow-core fibers constitute perhaps the most viable solution towards ultrafast, compact, and tunable lasers in the UV spectral region, which can even also extend into the mid-infrared (IR). Noise and spectral stability of such broadband sources are key parameters that define their true potential and suitability towards real-world applications. In order to investigate the spectral stability and noise levels in these fiber-based DUV sources, we generate an SC spectrum that extends from 180 nm (through phase-matched dispersive waves - DWs) to 4 μm by pumping an argon-filled hollow-core anti-resonant fiber at a mid-IR wavelength of 2.45 μm. We characterize the long-term stability of the source over several days and the pulse-to-pulse relative intensity noise (RIN) of the DW at 275 nm. The results indicate no sign of spectral degradation over 110 hours, but the RIN of the DW pulses at 275 nm is found to be as high as 33.3%. Numerical simulations were carried out to investigate the spectral distribution of the RIN and the results confirm the experimental measurements and that the poor noise performance is due to the high RIN of the mid-IR pump laser, which was hitherto not considered in numerical modelling of these sources. The results presented herein provide an important step towards an understanding of the noise mechanism underlying such complex light-gas nonlinear interactions and demonstrate the need for pump laser stabilization.

Relative intensity noise of InAs quantum dot lasers epitaxially grown on Ge

We report the relative intensity noise (RIN) characteristics of an InAs quantum dot (Qdot) laser epitaxially grown on the Ge substrate. It is found that the minimum RIN of the Ge-based Qdot laser is around −120 dB/Hz, which is 15 dB higher than that of a native GaAs-based Qdot laser with the same layer structure. The higher RIN in the Ge-based laser can be attributed to the high-density epitaxial defects of threading dislocations and antiphase domain boundaries.

WDM Orthogonal Subcarrier Multiplexing Based on Mode-Locked Lasers

The (de)modulation of broadband orthogonal subchannels relying on all-analogue signal processing potentially achieves high-capacity orthogonal subcarrier multiplexing (OSCM) electro-optical transceivers with low power consumption and latency. Overall transmission rates can be multiplied by employing wavelength division multiplexing (WDM) technology. Mode-locked lasers (MLL) are relevant optical frequency combs, as they produce a high number of comb tones while presenting a small footprint and low power consumption. Unlike baseband transmission, the use of high-frequency subchannels in OSCM systems overcomes the high relative intensity noise that MLL comb tones present at low frequencies. This paper reports a direct-detection real-time all-analogue WDM/OSCM experiment that emulates a 432 Gbit/s (20 × 21.6 Gbit/s) electro-optical transceiver. The 20 optical carriers were generated by a state-of-the-art quantum-dash MLL. The net data rate after considering forward error correction overheads is still higher than 400 Gbit/s. This is the highest capacity achieved in real-time broadband all-analogue WDM/OSCM links to date.

Mitigation of mode partition noise in quantum-dash Fabry-Perot mode-locked lasers using Manchester encoding and balanced detection.

We propose the use of Manchester encoding in conjunction with balanced detection to overcome the mode partition noise (MPN) limit of quantum-dash Fabry-Perot mode-locked lasers (QD-MLLs) used as multi-wavelength sources in short-reach applications. The proposed approach is demonstrated for a 10-mode laser, each carrying a 10-Gb/s signal. We show that bit-error-rate floors as high as 10-4 when traditional non-return-to-zero (NRZ) modulation is employed with a single-ended detection scheme can be pushed below 10-9 thanks to the introduction of Manchester encoding together with balanced detection. The benefit of the scheme could be attributed to the spectral shift of the Manchester spectrum, resulting in a smaller overlap with the high-relative intensity noise (RIN) region present at low frequencies, and the use of balanced detection. We clarify the origin of the performance improvement through comparisons of single-ended and balanced detection and the use of a RIN emulation technique. We unambiguously show that the use of balanced detection plays the leading role in MPN mitigation enabled by Manchester modulation.

Noise characterization of broadband fiber Cherenkov radiation as a visible-wavelength source for optical coherence tomography and two-photon fluorescence microscopy.

Optical sources in the visible region immediately adjacent to the near-infrared biological optical window are preferred in imaging techniques such as spectroscopic optical coherence tomography of endogenous absorptive molecules and two-photon fluorescence microscopy of intrinsic fluorophores. However, existing sources based on fiber supercontinuum generation are known to have high relative intensity noise and low spectral coherence, which may degrade imaging performance. Here we compare the optical noise and pulse compressibility of three high-power fiber Cherenkov radiation sources developed recently, and evaluate their potential to replace the existing supercontinuum sources in these imaging techniques.

Normalization detection scheme for high-speed optical frequency-domain imaging and reflectometry

We introduce a new signal detection method that can effectively suppress the effect of relative intensity noise (RIN) in optical frequency-domain reflectometry or imaging (OFDR/OFDI) schemes to enhance the sensitivity and dynamic range. In this method, spectral interferogram signal is normalized digitally by a spectral reference signal that contains the real-time spectrum and the RIN information of the frequency-swept source. Unlike the conventional balanced detection method that suppresses only additive intensity noises, we found that our proposed scheme removes both the additive and convolutional contributions of the RINs in the final interferogram signals. Experimental demonstrations were performed using a stretched-pulse optical coherence tomography (SP-OCT) system where the high RIN of a supercontinuum source had been a serious drawback. We have experimentally verified the superiority of our proposed scheme in terms of its improved dynamic range in comparison to the balanced detection method. In addition, we have shown that the noise suppression performance is immune to the spectral imbalance characteristics of the optical components used in the system, whereas the common-mode noise rejection of the conventional balanced detection method is influenced by them.

Experimental investigation of continuous-wave supercontinuum ring laser composed of clad-pumped Er/Yb codoped fiber and highly-nonlinear optical fiber

We experimentally demonstrate a practical benefit of our proposed continuous-wave (CW) supercontinuum (SC) generating ring laser scheme in terms of low-cost device implementation and then investigate its output intensity noise property, as a further study to our previous paper of Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599]. First, we implement a practical and low-cost CW SC laser using a double clad Er/Yb codoped fiber and a highly nonlinear dispersion-shifted fiber (HNL-DSF). Unlike the scheme in Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599] the gain medium of an Er/Yb fiber is clad-pumped by low quality multimode pump LDs to underline the fact that our proposed scheme does not require any high power single-mode laser pump. A SC of a bandwidth larger than 470nm is readily achieved. Next, we measure relative-intensity-noise (RIN) of the generated SC and compare it with that of a pumped Er/Yb fiber ASE. The SC laser is found to have a much higher RIN than the ASE due to the nonlinear amplification of quantum fluctuations both in the seed light oscillation and in the Raman scattering process.

A multiwavelength CW source based on longitudinal mode-carving of supercontinuum generated in fibers and noise performance

We experimentally demonstrate novel multiple wavelength continuous wave (CW) sources based on longitudinal mode-carving of supercontinuum (SC) generated in optical fibers. We show that by longitudinal mode-carving of the SC we can generate >600 wavelength channels with 10-GHz precise channel spacing and -6-dBm/ch power level at the >48-nm flat region (/spl plusmn/0.5-dB spectral uniformity) of the SC. By full utilization of the generated SC, the channel count exceeding 1600 can be accomplished. Moreover, we study the noise performance of the carved CW signals experimentally and theoretically. Experimentally, we measure the relative intensity noise (RIN) to characterize the noise performance of the generated CW signals. An average RIN value of -107 dB/Hz is obtained. Compared with the CW sources carved directly from the pump laser, a 4-dB/Hz RIN degradation is measured. The two main reasons behind the high RIN values are determined to be the frequency instability and the low side-mode suppression ratio of the pump laser. Experimentally, we confirm that the combined effect of the frequency instability and the low side-mode suppression ratio of the pump laser can degrade the RIN as much as /spl sim/30 dB/Hz. Theoretically, we estimate the lowest achievable RIN values as -160-dB/Hz and -144-dB/Hz RIN after the erbium-doped fiber amplifier (EDFA) and the SC fiber, respectively. These results indicate that, starting with a stable pump laser, CW lasers with <-140-dB/Hz RIN can be achieved.

Ultimate capacity of a laser diode in transporting multichannel M-QAM signals

In this paper, we investigate the ultimate M-ary quadrature amplitude modulated (M-QAM) channel capacity of a laser diode which is limited by the laser clipping induced nonlinear distortions. Our study includes a spectral analysis, a complete system simulation, and an experiment which used up to 70 channels of vector arbitrary waveform synthesizer generated quadrature phase shift keyed (QPSK) or 16-QAM signals to modulate an isolated/cooled distributed-feedback (DFB) laser and two unisolated/uncooled Fabry-Perot (FP) lasers, respectively. Our analytical results show that for an upstream laser diode, over 1000 QPSK channels or 170 16-QAM channels can be delivered, even in the presence of a high relative intensity noise (RIN) of -115 dB/Hz. However, these high capacities are reduced significantly when we consider the effect of collision-based medium access control (MAC) protocols. We found that, in the worst case condition (collisions occur in all but one channels), the ultimate QPSK channel capacity of an upstream laser diode is dramatically reduced from over 1000 to 125 for eight collisions/channel. These results have important implications to systems transporting frequency-stacked return-path bands with or without collision-based MAC channels. As regard to the ultimate capacity of a down-stream laser diode with a RIN level of -135 dB/Hz, we found that as high as 600 and 128 channels of 64-QAM and 256-QAM signals (equivalent to 3600 and 1152 channels of MPEG-II live video signals) can be transported, respectively.

Modal noise reduction of laser diodes with specific thickness coating

A low facet reflectivity laser diode is able to oscillate in multilongitudinal modes, and has a high relative intensity noise (RIN) and reduces mode hopping noise. By introducing the electron-cyclotron-resonance (ECR) plasma deposition method, a good facet coating film has been obtained. We have demonstrated that a specific coated laser has very low modal noise characteristics even under direct analogue intensity modulation.