Dual cavity laser

A dual cavity laser having a positively mode-locked structure capable of double increasing the repetition rate of output light. The dual cavity laser includes an amplitude modulator to simultaneously generate two modulating signals with opposite phases. The dual cavity laser has a simply modified structure from that of the conventional single cavity laser to have two resonators, so that it can have a repetition rate corresponding to two times that of the single cavity laser. One of the resonators may have an adjusted optical property so as to obtain two kinds of output optical pulses with different optical properties.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a dual cavity laser adapted to doubly 
increase the repetition rate of output light, and more particularly to a 
dual cavity laser having a positively mode-locked structure capable of 
double increasing the repetition rate of output light. 
2. Description of the Prior Art 
Typically, Fabry-Perot interferometers are interference systems including 
two parallel, partially reflecting mirrors. Each mirror has an optically 
perfect plane surface and a high reflectivity. When a light source is 
viewed through such an interference system, sharp interference patterns 
having the form of concentric circles are exhibited due to the multiple 
reflection of light between two mirrors. In particular, such an 
interference system is used to analyze the optical spectrum of light. 
On the other hand, Fabry-Perot resonators are optical resonators having a 
pair of reflecting surfaces arranged in such a manner that their optical 
axes are accurately aligned with each other. Each reflecting surface has a 
high reflectivity. Fabry-Perot resonators using spherical reflecting 
mirrors are usually used as optical resonators in most of lasers. 
In particular, confocal Fabry-Perot resonators are widely used in laser 
oscillators. Such confocal type Fabry-Perot resonators include a pair of 
identical spherical reflecting mirrors having a common focal point on 
their optical axes. 
FIG. 1A shows the arrangement of a conventional single cavity Fabry-Perot 
laser for active mode-locking. 
Typically, laser consists of three parts, namely, a gain medium, a pumping 
unit for a population inversion in the gain medium, and a resonator 
constituted by two parallel mirrors. In FIG. 1A, the reference numeral 11 
denotes an amplifier consisting of the gain medium and pumping unit. The 
resonator is indicated by a pair of mirrors M10 and M11. The amplitude 
modulator, which is denoted by the reference numeral 12, is disposed in 
the resonator in order to achieve an active mode locking. 
If one of two mirrors has a reflectivity less than 100% at the wavelength 
of oscillating light, the laser beam is usually coupled out from the 
resonator through that mirror. 
In order to increase the peak power of the laser by active mode-locking, 
the amplitude modulator 12 which periodically modulates the loss of light 
should be provided in the resonator. 
FIG. 1B shows a periodic loss of light induced in the resonator by the 
amplitude modulator with the periodicity of T. FIG. 1C shows the timing 
between the mode-locked optical pulses and the loss generated by the 
amplitude modulator. 
Referring to FIGS. 1B and 1C, it can be found that optical pulses are 
generated at the minima of the loss of light. 
The brief description of the principle of the mode locking is as follows. 
When the loss of light is periodically modulated, mode-locked pulses are 
generated at every minima of the periodic loss. Such a mode locking 
condition can be obtained by driving the amplitude modulator 12 at the 
round trip frequency(1/T Hz) of the resonator or its harmonic frequencies. 
In accordance with the principle of the active mode locking, optical pulses 
are generated at a repetition rate identical to the driving frequency of 
the amplitude modulator. As a result, it is impossible to obtain two kinds 
of output pulses with different optical properties from the conventional 
laser cavity. The conventional laser is also more or less inconvenient 
when it is applied to a variety of technical fields such as optical 
communications and optical sensors. 
SUMMARY OF THE INVENTION 
Therefore, an object of the invention is to provide a dual cavity laser 
including a dual resonator arrangement capable of not only doubly 
increasing the repetition rate of output pulses, but also obtaining two 
kinds of output pulses with different optical properties. 
In accordance with the present invention, this object is accomplished 
through a cavity laser comprising a pumping unit adapted to achieve 
population inversion of a gain medium, further comprising: first 
reflecting means for reflecting light; second reflecting means arranged in 
parallel to the first reflecting means; third reflecting means arranged in 
parallel to the first reflecting means; amplifying means arranged between 
the first and second reflecting means and between the first and third 
reflecting means, the amplifying means consisting of the gain medium and 
the pumping unit; and amplitude modulating means arranged between the 
first and second reflecting means and between the first and third 
reflecting means while being connected to the amplifying means in series 
to provide a pair of resonators, the amplitude modulating means adapted to 
periodically modulate a loss of light for the active mode-locking so as to 
increase the peak power of the laser output. 
In accordance with a preferred embodiment of the present invention, the 
independent resonators produce two kinds of optical pulses with the same 
repetition rate, but shift each other with a half period of each pulse 
train. 
Accordingly, where laser output is obtained through the reflecting means 
arranged at the side of the gain medium which the resonators hold in 
common, the repetition rate of output optical pulses becomes double the 
driving frequency of the amplitude modulating means. 
Where the optical properties of one of the resonators are appropriately 
adjusted, relative characteristics of two output pulses become different 
from each other. For example, when the birefringence of one resonator is 
changed, it is possible to obtain two kinds of optical pulses with 
different states of polarization. Thus, it is possible to generate two 
kinds of optical pulses with different characteristics as optical 
properties of the resonators are appropriately adjusted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2A shows a dual cavity Fabry-Perot laser according to the present 
invention. 
As shown in FIG. 2A, the dual cavity laser of the present invention 
includes an amplitude modulator 22 for simultaneously generating two 
modulating signals with opposite phases, an amplifier 21 as a gain medium, 
and at least three reflecting means, for example, mirrors constituting a 
dual resonator. 
The gain medium 21 is disposed at the input terminal side of the amplitude 
modulator 22. The mirror M20 is coupled to the gain medium 21 whereas the 
remaining mirrors M21 and M22 are coupled to two output terminals of the 
amplitude modulator 22, respectively. 
A selected one of the three mirrors has a reflectivity less than 100% so 
that output light can be obtained through the selected mirror. 
In order to increase the pulse repetition rate, output pulses should be 
obtained through the mirror M20 disposed at the side of the gain medium 
21. 
In FIG. 2A, the arrows indicated within the resonator express the paths of 
light beams oscillating in the resonator. 
In accordance with the present invention, a dual resonator structure is 
obtained. One resonator consists of the mirror M20, amplifier 21, 
amplitude modulator 22 and mirror M21 whereas the other resonator consists 
of the mirror M20, amplifier 21, amplitude modulator 22 and mirror M22. 
The amplitude modulator 22 used in the dual cavity laser should 
simultaneously generate two modulating signals with opposite phases. That 
is, the mirror M20, amplifier 21, amplitude modulator 22 and mirror M21 
constitute one resonator whereas the mirror M20, amplifier 21, amplitude 
modulator 22 and mirror M22 constitute another resonator. In other words, 
the dual resonator has a structure consisting of combined two resonators 
as shown in FIG. 1A. 
In order to simultaneously obtain a mode locking in the two resonators by 
the single amplitude modulator 22, the lengths of the resonators should be 
the same. Alternatively, the length of one resonator should correspond to 
a multiple of the length of the other resonator. 
FIG. 2B shows the relationship between the amplitude modulating signal 
relating to a mode locking of one of the two resonators, namely, loss of 
light (indicated by the dotted line in the figure) and optical pulses 
generated due to the amplitude modulating signal. 
FIG. 2C shows the relationship between the amplitude modulating signal 
relating to a mode locking of the other resonators, namely, loss of light 
(indicated by the dotted line in the figure) and optical pulses generated 
due to the amplitude modulating signal. 
Referring to FIGS. 2B and 2C, it can be found that the phases of two 
amplitude modulating signals depicted by the dotted lines are opposite to 
each other. 
The resonators have the same repetition rate because their mode locking 
occurs by the same amplitude modulator 22. 
However, optical pulses respectively generated from the resonators have a 
time difference corresponding to a half cycle (T/2) of the amplitude 
modulating signals because the phases of the two amplitude modulating 
signals, namely, loss of light depicted by the dotted lines are opposite 
to each other and because the mode-locked optical pulses are generated at 
the minima of the loss of light is minimized. 
Where the mirror M21 or M22 is used as an output mirror of the dual cavity 
laser, it is possible to obtain a train of pulses as shown in FIGS. 2B or 
2C. Each pulse train has the same repetition rate as that of the single 
cavity laser as shown in FIG. 1C. 
These two kinds of optical pulses have unique optical characteristics, 
respectively, because they oscillate in different resonators while being 
generated from the same gain medium. For example, although they have the 
same coherence length because they are generated from the same gain 
medium, they have different states of polarization respectively. The 
polarized state of output optical pulses is determined by the 
birefringence of the laser cavity. Accordingly, if the birefringence of 
one of the resonators is changed at a portion of the laser where two 
resonators don't hold in common, it is possible to obtain two kinds of 
independent optical pulses with different states of polarization. 
FIG. 2D illustrates a train of mode-locked output optical pulses emitted 
from the dual cavity laser. Referring to FIG. 2D, it can be found that the 
output optical pulses of FIG. 2D consist of the optical pulses of FIGS. 2B 
and 2C mixed in an alternating manner. As a result, the output optical 
pulses generated from the dual cavity laser structure exhibit a repetition 
rate corresponding to two times that of the single cavity laser structure. 
The optical pulse train of FIG. 2D is a train of mode-locked optical pulses 
emitted when the mirror M20 of the dual cavity laser is used as an output 
mirror. 
Two kinds of optical pulses respectively generated from two resonators, 
namely, the optical pulses of FIGS. 2B and 2C are positioned in an 
alternating manner with intervals of T/2. 
Therefore, the dual cavity laser structure has a repetition rate of output 
optical pulses corresponding to two times that of the conventional single 
cavity laser structure. 
Consequently, actively mode-locked dual cavity laser can obtain two kinds 
of output optical pulses with different characteristics by appropriate 
selection of an output mirror. 
Where the mirrors M21 and M22 of FIG. 2A are selected as output mirrors, 
optical pulses are generated at the same repetition rate of the single 
cavity laser. On the other hand, where the mirror M20 is selected as an 
output mirror, optical pulses are generated at a repetition rate 
corresponding to two times that of the single cavity laser. 
It is also possible to obtain two kinds of optical pulses exhibiting 
different characteristics by varying optical properties of one of the 
resonators. 
As apparent from the above description, in accordance with the present 
invention, it is possible to obtain two kinds of output pulses exhibiting 
different optical properties while doubly increasing the repetition rate 
of the output light by a simple modification in laser structure. The 
repetition rate of optical pulses is directly associated with the capacity 
of transferable information in the case of optical communications. Where 
the laser is used as a light source for optical sensors, the repetition 
rate of optical pulses is closely associated with the measurement 
sensitivity. 
In this regard, the dual cavity laser of the present invention, which 
exhibits a repetition rate corresponding to two times that of the 
conventional active mode locking structure can be used as a very efficient 
pulse source in various technical fields associated with, for example, 
optical communications and optical sensors. 
Although the preferred embodiments of the invention have been disclosed for 
illustrative purposes, those skilled in the art will appreciate that 
various modifications, additions and substitutions are possible, without 
departing from the scope and spirit of the invention as disclosed in the 
accompanying claims.