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Generating short pulses = mode-locking

Locking the phases of the laser modes yields an ultrashort pulse.

A generic ultrashort-pulse laser

A generic ultrafast laser has a broadband gain medium, a pulse-shortening device, and two

or more mirrors:

An MLLD (Mode-locked Laser Diodes) is a promising device as a light source for all-

optical processing systems because it has excellent features of a semiconductor laser and

mode-locking technique. There are two types of MLLD differentiated by the modulationmethods.

Active mode locking for electrical modulation.

Passive mode locking for optical modulation.

In an active MLLD, the mode-locking process is forcibly induced by external modulationand an output pulse train is synchronized with the external signal.

Passive MLLD needs no external modulation source for pulse generation and the

characteristics do not suffer from electrical limitation, whereas the repetition frequency isnot synchronized with external electrical signals.

In order to solve the above problem during practical use, hybrid mode locking and optical

synchronous mode locking have been developed.

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From the point of view of cavity configuration, MLLDs can be categorized into two types,

namely external cavity and monolithic types.

An external-cavity MLLD has the advantages that usable functions such as a wavelengthtuning and repetition frequency tuning can be added by inserting optical components into

the cavity.

A monolithic MLLD, in contrast, has the advantage of a smaller size and mechanicalstability as compared with an external cavity MLLD.

Active Mode LockingActive mode locking arises from the electrical modulation of gain or loss in a laser cavity

at a frequency near the round-trip frequency.

Schematic cross-section and driving method of an active MLLD

In this case, a laser diode is integrated with an electroabsorption (EA) modulator in thecavity. Another feature of active mode locking is that it modulates directly to a part of the

gain section without an EA modulator.The round-trip frequency fr of a laser diode is given in the following expression:

where c is the velocity of light in a vacuum, ng is the refractive index for the group velocityin the cavity, and L is the cavity length.

When a RF electric field at the frequency of fM near fr is applied to the electro-absorption

layer, the absorption coefficient is described by:

where 0 is a constant, and M is the modulation coefficient, and fM is the modulation

frequency. The repetition frequency is equal to fM, and close to the round-trip frequency.According to Hauss theory [38], the output pulse shape of active mode locking becomes a

Gaussian waveform, and the pulse width is given by

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where g is the gain of the peak, fL is the bandwidth of the gain, and the dispersion of the

resonator is disregarded. It is an important relationship that the pulse width is inversely

proportional to the fourth root of the modulation index M.

Passive Mode Locking

Device structure for monolithic type of passive MLLD

In the case of passive mode locking, a saturable absorber (SA) is substituted for the EAmodulator of an active MLLD. A multiple quantum well (MQW) structure of same

material as the gain section is ordinarily used as an SA.When currents are injected into a

gain section and an SA is reverse biased, passive mode locking occurs.Note that the pulse generation can only be obtained by DC bias without high-speed

electronics. Generally, an SA is placed at a resonator edge where a light pulse arrives once

in every round-trip. In contrast, a configuration where an SA is located at the center of alaser cavity is called a colliding pulse mode-locked laser diode (CPM-LD). The CPM can

reduce the absorption saturation energy equivalently by coherent interaction between

incoming optical pulses from both sides of the gain section at the SA.

The mode-locked frequency fML for passive mode locking is N times round-trip frequency(where N is an integer).

It is called fundamental mode locking when N =1 and Nth order harmonic mode locking

when N =2.

Above figure illustrates a case where an SA is placed in the resonator edge and N =1, and a

case where CPM configuration usually becomes N =2. In addition, higher order harmonicmode locking can be performed by putting an SA at a suitable position utilizing a CPM

effect.

According to Hauss theory, the pulse shape of passive mode locking becomes a sech 2

waveform, and the pulse width is given by:

where Ep is the pulse energy, EA is the absorption saturation energy, EL is the gain

saturation energy, q(i) is the saturable absorption normalized by the cavity loss before

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passage of the pulse, g(i) is gain normalized by the cavity loss before passage of the pulse,

and fc is the gain bandwidth.

The pulse width is narrower, as the pulse energy is larger, the saturation energy is smaller,and nonlinear absorption is larger. This means that passive mode-locking characteristics

depend on gain and absorption modulation depth and on the intracavity pulse energy.

This is the necessary condition for the absorption saturation energy to be smaller than the

gain saturation energy.

Hybrid Mode Locking:

Schematic cross-section and driving method of a hybrid MLLD

RF signal when a frequency around the round-trip frequency of the passive MLLD is

applied to the SA.