Optimizing Coherence Suppression in a Laser Broadened by Phase Modulation With Noise

Abstract

Phase noise can be modulated onto the output of a laser with an electro-optic phase modulator (EOM) to create a highly incoherent broadened source with low intensity noise. This technique leaves a small but finite fraction of the coherent carrier power that can be highly detrimental in applications requiring incoherent light. This paper shows that the carrier suppression in a laser broadened by this technique can be calculated for an arbitrary noise probability density function. The carrier suppression can be varied experimentally by adjusting the noise voltage standard deviation Gvσ and saturation voltage Vsat of the amplifier that amplifies the noise source that drives the EOM. Simulations show that suppressions better than -30 dB are attainable for reasonable tolerances in Gvσ, for example an EOM with a Vπ of 4.7 V, a Vsat of 6.3 V, and Gvσ/Vπ = 0.73 ± 0.03. Even greater tolerances can be achieved with a higher Vsat (21 V), lower Vp (3.5 V), and Gvσ/Vπ ≈ 3.2 ± 2.3. This model aids in selecting these parameters such that the carrier suppression is resilient to fluctuations in these voltages due to temperature variations and aging of the components. The model predictions are validated by testing a fiber optic gyroscope interrogated with a broadened laser for various Gvσ and Vsat and inferring the carrier suppression from its measured noise. A -44-dB carrier suppression was observed for a nonlinear amplifier with Gvσ = 3.43 V, Vsat = 6.3 V, and an EOM with Vπ = 4.7 V, in agreement with predictions.

Ultrashort Pulse Generation in Modeless Laser Cavity

“…Generally, the suppression increases with modulation amplitude, though not necessarily monotonically. Modulation with noise nearly completely suppressed the carrier for a phase modulation amplitude of 0.72 π (measured as a standard deviation) [26]. Since the modulation was aperiodic and random, the resulting spectrum is continuous.…”

Fiber optic gyroscope interrogated with three multiplexed broadened semiconductor lasers

“…To this end, the laser has been broadened by modulating its phase externally with either sine waves, [3] a pseudo-random bit sequence, [4] or broadband noise. [5,6,7] This last approach has been the most successful so far. The laser light is sent through an electro-optic phase modulator driven by broadband noise generated by amplifying the thermal noise of a resistor with a chain of high-speed amplifiers.…”

“…With proper selection of the voltage relative to both the V of the modulator and the saturation voltage of the last amplifier in the chain, a laser can be broadened to several times the bandwidth of the noise while minimizing the residual carrier in the broadened signal. [7] Minimizing the residual carrier is critical because being highly coherent, even a small residual-carrier power can induce a significant noise and drift due to backscattering and polarization coupling. [7] These values are sufficient to eliminate most of the nonreciprocal noise and drift, but the residual Kerr effect still needs to be further suppressed by modulating the input laser power with a 50% duty cycle.…”

“…[7] Minimizing the residual carrier is critical because being highly coherent, even a small residual-carrier power can induce a significant noise and drift due to backscattering and polarization coupling. [7] These values are sufficient to eliminate most of the nonreciprocal noise and drift, but the residual Kerr effect still needs to be further suppressed by modulating the input laser power with a 50% duty cycle. [1,6] These combined improvements led recently to the demonstration of a 3-km laser-driven FOG with a noise as low as 900 µdeg/√h, a drift of 12.6 mdeg/h, [6] and a mean-wavelength stability of less than 1 part per million.…”

1030 nm All-Fiber Closed-Loop Fiber Optic Gyroscope with High Sensitivity

“…Therefore, new techniques and sources are mandatory in order to further improve the gyroscope performance. For example, IFOG driven by 1550 nm laser source broadened by phase modulation used as new light source was developed in order to minimize ARW by 3 dB [15,16]. This technique has some drawbacks such as requirement for additional component that increase size and complexity in IFOG architecture.…”