Analysis of temporal pulse development in passively mode-locked lasers

Abstract

Abstract. Pulse-shortening and pulse broadening effects in passively mode-locked lasers are analysed. A steady-state pulse duration limit is calculated and compared with round-trip simulations. The numerical calculations apply to a picosecond Nd: glass laser. Methods of short-pulse generation are discussed. 42.55R, 42.60D, 42.65 In passively mode-locked pulsed lasers the statistical spontaneous emission bursts have a duration approximately equal to the inverse of the fluorescence linewidth. In the linear laser phase (constant dye absorption) the finite spectral gain profile of the active medium narrows the spectral width and broadens the duration of the circulating spikes. In the nonlinear laser phase (nonlinear dye transmission, mode-locking region) the most intense spike is preferentially amplified and strong background discrimination is achieved. Pulse shortening occurs in the saturable absorber cell. (For reviews on mode-locking see [1][2][3][4][5][6][7].) The natural mode selection in the active medium acts against the shortening. Intensity dependent loss mechanisms like two-photon absorption in Nd: glass rods [8] broaden the pulse duration at high laser intensities [9]. In this paper we analyse the pulse shortening and pulse broadening effects in passively mode-locked lasers. Intensity-dependent stationary pulse durations are found by equating pulse shortening and pulse broadening per round-trip in the laser oscillator. In the mode-locking region the transient temporal development of a picosecond pulse with constant background level is simulated and the approach to the stationary situation is studied. The theory presented applies to all passively modelocked lasers, either pulsed or continuously working [10]. The numerical calculations in this paper apply to a passively mode-locked Nd: glass laser. The calculations reveal conditions for optimum shortpulse generation in the oscillator. Preliminary measurements with a Pockels cell intensity limiter in the resonator led to pulse shortening in agreement with the theory. PACS: Stationary Pulse DurationsThe changes of pulse duration per round-trip in the oscillator are analysed. Pulses are broadened by the finite spectral width of the gain and by intensity dependent nonlinear losses. Here pulse broadening by two-photon absorption in Nd: glass rods is discussed. Pulse shortening occurs in the nonlinear transmission region of the saturable absorber. The pulse duration ratio f$ = At L0 /At Lb where At L0 is the pulse duration after n +1 round-trips and At Li the pulse duration after n round-trips, is given byj8 GAIN is the ratio of pulse broadening by the finite spectral width of the gain profile. /J XPA represents the ratio of pulse broadening by two-photon absorption in the active medium. /J DYE is the pulse shortening ratio of the saturable dye.The spectral gain profile of the active medium approximately has the form [9]

Intra-cavity pulse compression in passively mode-locked Nd-glass lasers

“…3a the spectral shape gp(co) of the flash-lamp excited fluorescence emission of the Nd-phosphate glass rod is included (dotted curve). This fluorescence profile determines the gain profile ^Q(OJ) of the active medium [4]. The effective gain profile of the oscillator is given by g(co) = gQ(oS) T B (co).…”

“…In passively mode-locked Nd-phosphate glass lasers nearly bandwidth limited pulses of 5 to 7 ps duration are generated in the rising part of the pulse train [1][2][3][4]. The spectral width of the pulses (A?…”

Passive Q-switching and mode-locking for the generation of nanosecond to femtosecond pulses

“…The obtained pulse durations are of the order of the absorption recovery time. In a steady-state system the pulse duration is determined by the balance between pulse shortening (saturable absorber) and pulse broadening (finite spectral width of active medium, power limiting effects [59,113]) [32,112].…”

“…In this case the nonlinear transmission period is too short to approach the steady-state situation, and the pulse duration remains longer than the steady-state limit. A linear loss feedback may keep the mode-locked pulse intensity in the nonlinear transmission region, prolong the pulse train length, and shorten the pulses down to the steady-state limit [112,[135][136][137].…”

“…For very fast saturable absorbers the mode-locking threshold is difficult to approach since the peak spiking intensity of the laser remains below the saturation intensity [32] and special focusing geometries are necessary to achieve mode-locking [147,170]. The use of two saturable absorbers of widely different saturation intensity (different x A ) offers the possibility of low mode-locking threshold and short pulse generation [112,171].…”