Measurement of mode-locked laser timing jitter by use of phase-encoded optical sampling

Abstract

The phase-noise characteristics of a harmonically mode-locked fiber laser are investigated with a new measurement technique called phase-encoded optical sampling. A polarization-maintaining ring laser is mode locked by use of the short-pulse electrical output of a resonant-tunneling diode oscillator, enabling it to produce 30-ps pulses at a 208-MHz repetition rate. The interferometric phase-encoded sampling technique provides 60-dB suppression of amplitude-jitter noise and allows supermode phase noise to be observed and quantified. The white-noise pulse-to-pulse timing jitter and the rms supermode timing jitter of the laser are measured to be less than 50 and 70 fs, respectively.

Optically sampled analog-to-digital converters

“…The phase-encoded optical sampling technique has also enabled a new tool for characterizing the timing jitter of shortpulse optical sources [11]. Since the technique provides extraordinary suppression (60 dB) of laser amplitude noise, it allows the laser phase noise or timing jitter to be independently observed and quantified.…”

“…This implies that the phase-encoded sampling technique is insensitive to laser amplitude noise. Indeed, the amplitude noise suppression of this technique has been measured to be at least 60 dB [11]. Third, small deviations of the modulator bias point from quadrature ( ) cause only a dc offset error and do not degrade the linearity, as in the case of the single-output digital linearization approach [45].…”

“…The remaining phase-noise component can introduce periodic pattern spurs when high-frequency signals are sampled. However, signal frequencies as high as 3 GHz have been sampled using the current laser without degrading the SFDR or the SNR [11].…”

“…The calculation assumes that timing jitter is the only noise source. The present-day electronic and optical sampling-aperture jitter ranges (hatched areas) are from [1], [10], and [11].…”

“…The principal advantages of optical sampling are: 1) modern mode-locked lasers can produce high-frequency ( 10 GHz) periodic sequences of optical pulses with timing jitter significantly below that of electronic circuitry, and 2) the sampling process can be made to be highly linear with negligible back-coupling between the optical sampling pulses and the electrical signal being sampled. The timing jitter of actively mode-locked fiber lasers has been measured to be 10 fs over narrow-band integration limits (100 Hz to 1 MHz) [10] and 50 fs pulse to pulse [11] (see hatched region in Fig. 1).…”