Enhanced mode locking of color-center lasers

The development and application of femtosecond laser systems

Some background as well as recent progress in the development of femtosecond lasers are discussed together with a brief outline of a few representative emergent applications in biology and medicine that are underpinned by access to such sources. We also provide a short summary of other contributions in this focus issue.

Frontiers in Ultrashort Pulse Generation: Pushing the Limits in Linear and Nonlinear Optics

Optical pulses in the 5-femtosecond range are produced by a variety of methods. Although different in technical detail, each method relies on the same three key components: spectral broadening due to the nonlinear optical Kerr effect, dispersion control, and ultrabroadband amplification. The state of the art of ultrashort pulse generation is reviewed with a focus on direct laser oscillator schemes.

All-solid-state self-mode locking of a Nd:YLF laser

An all-solid-state, Kerr-lens mode-locked Nd:YLF laser is reported that uses a simple X-fold cavity with separate gain and nonlinear media. Self-starting pulses of 3 ps are reported at 1.047 microm with significant enhancements in stability and starting behavior over previous Kerr-lens mode-locked Nd:YLF lasers. The benefits of this X-fold cavity design for general passive mode locking of laser-diode-pumped solid-state lasers are also addressed.

Kerr lens effects in a ring resonator with an aperture:mode locking and unidirectional operation

A ring resonator containing a Kerr lens and a Gaussian slit is analyzed. From the point of view of Kerr lens mode locking, it is shown that a self-defocusing nonlinear element is as effective as a self-focusing one and that the positioning of the Kerr element introduces trade-offs between self-starting and operational stabilization. The nonlinearity can lead to unidirectional lasing.

Femtosecond pulses with high energies from a coupled-cavity saturable-absorber mode-locked KCl F(A)(Tl) color-center laser

By using a bulk InGaAsP saturable absorber, we passively mode lock a KCl F(A)(Tl) color-center laser in a resonator configuration where the gain material is placed in a main cavity and the saturable absorber is placed in a weakly coupled external cavity. We obtain self-starting and self-stabilized resonant passive mode-locked laser operation, either with pulses as short as ~320 fs or average output power as high as ~380 mW. The pulse width-bandwidth product is ~0.47, with pulse energies of as much as 2.5 nJ. By temperature tuning the saturable absorber band edge, we realize laser operation with similar performance over the wavelength range of 1.50-1.55 microm. When compared with mode-locked KCl F(A)(Tl) laser operation, where both the gain material and the saturable absorber are placed in a common single cavity, the resonant passive mode-locking technique yields pulses with only moderately increased widths (by ~50 fs) but significantly higher energies (by a factor of ~4).

Femtosecond pulse generation by using an additive-pulse mode-locked chromium-doped forsterite laser operated at 77 K

Using an acousto-optically mode-locked chromium-doped forsterite laser, operated at 77 K and coupled to a nonlinear resonator containing a single-mode fiber, we have produced femtosecond pulses of 150-fs duration at 1.23 microm with useful output powers of approximately 60 mW This represents what is to our knowledge the first demonstration of femtosecond pulse generation from this laser system using the coupled-cavity mode-locking scheme.

Solid-state low-loss intracavity saturable absorber for Nd:YLF lasers: an antiresonant semiconductor Fabry-Perot saturable absorber

We introduce a new low-loss fast intracavity semiconductor Fabry-Perot saturable absorber operated at anti-resonance both to start and sustain stable mode locking of a cw-pumped Nd:YLF laser. We achieved a 3.3-ps pulse duration at a 220-MHz repetition rate. The average output power was 700 mW with 2 W of cw pump power from a Ti:sapphire laser. At pump powers of less than 1.6 W the laser self-Q switches and produces 4-ps pulses within a 1.4-micros Q-switched pulse at an approximately 150-kHz repetition rate determined by the relaxation oscillation of the Nd:YLF laser. Both modes of operation are stable. In terms of coupled-cavity mode locking, the intra-cavity antiresonant Fabry-Perot saturable absorber corresponds to monolithic resonant passive mode locking.

Femtosecond pulse generation in Ti:Al(2)O(3) using a microdot mirror mode locker

A new nonlinear intracavity device called a microdot mirror mode locker is demonstrated for ultrashort-pulse generation in solid-state lasers. The microdot mirror produces fast saturable absorber action by using self-focusing. Pulse durations of 190 fs are achieved in a Ti:A1(2)O(3) laser. The microdot mirror mode locker is modular, preserves wavelength tunability, and can be extended to a wide range of solid-state lasers.

Noncritical matching of cavity lengths for uninterrupted additive-pulse mode locking of a cw Nd:YLF laser

Stable interferometrically insensitive operation of an additive-pulse mode-locked Nd:YLF laser is observed over 4.8 microm of continuous detuning of the cavity lengths. This phenomenon continuously persists over 250 microm of coarse cavity-length detuning. Outside this range interferometric mode locking typically associated with additive-pulse mode locking is observed for detuning lengths of over 1 mm. The external cavity of the system consists of a multimode fiber whose four possible second-order proper modes are all excited. The underlying mechanism is a slow process with response times in the subhertz and hertz regions.

Self-starting of passively mode-locked lasers

The self-starting condition of passively mode-locked lasers, including those locked by additive-pulse mode locking, is reexamined. A quantitative model is developed for a threshold intensity for self-starting based on the hypothesis that spurious reflections have to be overcome by coherent injection signals of sufficient amplitude.

Kilohertz-rate continuum generation by amplification of femtosecond pulses near 1.5 microm

A NaCl color-center amplifier is used to amplify femtosecond pulses from an additive-pulse mode-locked laser at wavelengths between 1.52 and 1.60 microm. Pulse energies of several microjoules are obtained with pulse widths as short as 100 fs at kilohertz repetition rates. These pulses have been used to generate a continuum in a variety of solid and liquid media for hours without optical damage. The continuum generated in BaF(2) covers the wavelength range of 400 nm < lambda < 3.5 microm.

Theory of passive additive-pulse mode locking

The laser configuration of newly developed additive-pulse mode locking, also known as coupled-cavity mode locking, can be viewed as an intracavity interferometer. By solving the equation of motion of the two coupled cavities, a mathematical description of the self-starting mechanism is obtained. With this method, the transient pulse evolution of an initial seed pulse can be calculated and thereby optimized. The structure of the equation of motion suggests new single-cavity configurations of additive-pulse mode locking, and the same method of analysis can be applied to them.

Self-starting of an additive-pulse mode-locked color-center laser

Self-starting of additive-pulse mode-locking (APM) behavior has been observed in a NaCl:OH color-center APM laser over the wavelength range of 1.52-1.60 microm. Pulses as short as 160 fs have been produced at a wavelength of 1.55 microm in a stable pulse train. High-power levels in the fiber are required for the mode locking to self-start. The large fiber power makes it difficult to stabilize the cavity length by conventional methods: The feedback signal must now be derived from the second harmonic of the laser output. The self-starting APM laser exhibits less noise than the synchronously pumped APM laser because of the lack of pulse walk-off effects that are characteristic of the synchronously pumped APM laser.

Coupled-cavity resonant passive mode-locked Nd:yttrium lithium fluoride laser

We report coupled-cavity resonant passive mode locking of a Nd:YLF laser. This technique has produced 4-ps pulses at a wavelength of 1.047 microm with 390-mW average power at a 250-MHz repetition rate, corresponding to a 1.6-nJ pulse energy. The Nd:YLF rod was pumped with 1.5 W of power at 798 nm from a Ti:sapphire laser. The nonlinear reflector used in the coupled cavity was an InGaAs/GaAs strained layer multiple-quantum-well sample.

Passive coupled-cavity mode-locked color-center lasers

Stable, self-starting operation of a coupled-cavity mode-locked KCl:T1(0)(1) color-center laser has been achieved by incorporating a semiconductor diode optical amplifier into the control cavity. By adjustment of the amplifier drive current, nonlinearity in the regimes of either saturable gain or saturable absorption has been exploited to generate subpicosecond pulses. Under conditions of saturable gain, pulse durations as short as 280 fs were obtained. Self-starting has also been demonstrated for a coupled-cavity mode-locked NaCl:OH(-) color-center laser by using the same technique.

Passive mode locking with a nonlinear external coupled cavity at high pulse repetition rates

A general consideration is presented for obtaining high repetition rates in passively mode-locked lasers with a nonlinear external coupled cavity. Without modification of the main cavity, as much as a sixfold increase over the original repetition rate of 76 MHz has been obtained in a coupled-cavity passively mode-locked Nd:YLF laser. At a constant average output power, the pulse width increases linearly, while the detuning range decreases with increasing repetition rates.

Characteristics of an actively mode-locked 2-psec Ti:sapphire laser operating in the 1-microm wavelength regime

We examine the performance of an actively mode-locked Ti:sapphire laser operating in the 1-microm wavelength regime. Pulse durations of 1.7 psec are produced with an average output power of 400 mW. With the use of external dispersion compensation this pulse width has been further reduced to 850 fsec.

Coupled-cavity resonant passive mode-locked Ti:sapphire laser

We use a nonlinear quantum-well reflective sample in a coupled-cavity configuration for resonant passive mode locking of a Ti:sapphire laser, producing tunable pulses as short as 2 psec. Pulses of less than 10-psec duration are observed over a 50-nm wavelength tuning range using a single-quantum-well reflector, over approximately 2 mm of external cavity length detuning. Stable mode-locked pulse trains are obtained without active cavity length control; however, the optical spectrum depends on the phase. We generate up to the sixth harmonic of the fundamental repetition rate, at 1.2 GHz, by simply decreasing the external cavity length.

Starting dynamics of additive-pulse mode locking in the Ti:A1(2)O(3) laser

The starting dynamics of the additive-pulse mode-locked Ti:AI(2)O(3) laser are investigated. Mode locking develops from mode beating with pulse formation times of several hundred microseconds. A simple model based on additivepulse mode-locking theory is developed that describes self-starting and pulse evolution.

Self-starting additive-pulse mode locking of a Nd:glass laser

Self-starting cw mode-locked operation of a Nd:glass laser has been achieved by using a nonlinear external resonator. The coupled cavity incorporating an optical fiber initiates and sustains ultrashort-pulse generation, resulting in a stable train of 380-fsec pulses.

Noise characterization of femtosecond color-center lasers

We used power-spectrum techniques to measure both amplitude noise and timing jitter of two femtosecond colorcenter lasers in the near infrared: a passively mode-locked laser and an additive-pulse mode-locked laser. The cw pumped passively mode-locked color-center laser showed better noise performance than the feedback-controlled, much more complex additive-pulse mode-locked color-center laser. For both laser systems, we observed unexpected drastic increases of amplitude noise under certain operation conditions. In the low-noise operation regime, the amplitude noise of the passively mode-locked color-center laser is entirely dominated by its pump laser, whereas the additive-pulse mode-locked laser exhibits >10 dB of excess noise up to 200 kHz and tended to break up in the subhertz regime. The timing jitter for both lasers was approximately 9 psec rms for jitter rates above 130 Hz.

Passive laser mode locking with an antiresonant nonlinear mirror

A nonlinear mirror that is based on an antiresonant ring incorporating a nonlinear element sensitive to the polarization state of the circulating light has been developed. No phase matching of the mirror to the laser cavity is necessary, so the mirror does not need active length stabilization. Spontaneous mode locking occurs when the mirror is incorporated into a cw Nd:YAG laser. Pulses of 11-psec duration are produced at 1064 nm; two output beams at 532 nm are also generated. An analysis indicates that the primary mechanism responsible for passive mode locking is self-phase modulation in the nonlinear element.

Subpicosecond pulse generation from a Nd:glass laser using a nonlinear external cavity

An actively mode-locked, continuous-wave Nd:glass laser pumped by a krypton laser at 0.8 microm has been coupled to a nonlinear external resonator. The 20-psec pulses produced by active mode locking are shortened to less than 1 psec with the nonlinear external cavity feedback. Subpicosecond pulse generation has been achieved at a pump power as low as 500 mW.

Passive mode locking of a cw Nd:YLF laser with a nonlinear external coupled cavity

Using an optical fiber as the nonlinear medium in an external cavity, we demonstrated passive mode locking of a cw Nd:YLF laser at a 1.053-microm wavelength. Nearly transform-limited pulses as short as 3.7 psec were obtained at a repetition rate of 76 MHz directly from the laser output without further compression. The average output power ranged from 1 to 7 W.

Self-starting additive-pulse mode locking of a Nd:YAG laser

We report additive-pulse mode locking of lamp-pumped Nd:YAG lasers at both 1.06 and 1.32 microm. Under completely self-starting conditions, stable cw mode-locked trains of 6-psec pulses have been achieved at an average output power of 2.4 W.

Self-starting additive-pulse mode-locked diode-pumped Nd:YAG laser

A diode-pumped Nd:YAG laser is demonstrated by using self-starting additive-pulse mode locking with a nonlinear external cavity. Pulse durations of 1.7 psec are generated at an average output power of 25 mW without the need for active amplitude or phase modulation. Design and scaling issues for this technology are discussed.

Mode locking of a continuous-wave titanium-doped sapphire laser using a linear external cavity

The mode locking of a Ti:Al(2)O(3) laser with a moving extracavity mirror is described. With no active mode-locking element and with no nonlinear element in the external cavity, pulses of less than 40 psec have been directly generated from a cw laser. A possible mechanism for this novel mode-locking technique is discussed.

Self-starting condition for additive-pulse mode-locked lasers

We analyze self-starting in additive-pulse mode-locked lasers and find that dynamic gain saturation can play a determining role. A simple condition for self-starting is presented for comparison with experiments on different laser systems.

Femtosecond passively mode-locked Ti:Al(2)O(3) laser with a nonlinear external cavity

Ultrashort pulses are generated in a Ti:Al(2)O(3) laser by using a coupled nonlinear external cavity. The external cavity uses self-phase modulation in an optical fiber to achieve passive mode locking without the need for synchronous pumping or acousto-optic modulation. A stable train of chirped 1.4-psec pulses is generated. After dispersive compensation, pulses as short as 200 fsec are obtained.

Interferential mode locking: Gaussian pulse analysis

A simple analysis of the interference between two Gaussian pulses is presented. It is shown that the resulting pulse may be shorter than the initial ones if the interference phase is properly adjusted. The compression effect results from the difference of the durations of the interfering pulses and from their phase modulation. This mechanism may be implemented in mode-locked lasers, eitherby adjunction of a coupled cavity or use of a pulse-shaping Michelson interferometer as the outputcoupler. Gain-bandwidth-limited pulses may then be obtained. The model provides analytical expressions for pulse duration.