Coherent light generators – Particular beam control device – Mode locking
Reexamination Certificate
1999-12-28
2004-11-09
Wong, Don (Department: 2828)
Coherent light generators
Particular beam control device
Mode locking
C372S026000, C372S019000, C372S021000, C372S030000, C372S098000
Reexamination Certificate
active
06816515
ABSTRACT:
This application claims priority from Korean Application No. 1998-15214, filed on Apr. 28, 1998, which claims priority from PCT/KR99/00202, filed Apr. 28, 1999, which are hereby incorporated by reference.
TECHNICAL FIELD
The present invention relates to a laser and a method for its operation, more particularly to a wavelength-swept pulse laser producing a short pulsed output with a center wavelength continuously varying with time and a method for generating such wavelength-swept laser pulses.
The present invention also relates to a wavelength-swept laser producing continuous output and a method for generating such wavelength-swept continuous laser light.
BACKGROUND ART
A wavelength-swept laser is a light source whose output wavelength continuously varies with time. Wide-band gain medium and wavelength sweeping means are required to operate such a wavelength-swept laser. The most efficient wavelength sweeping method is to vary the center wavelength of a wavelength tunable filter placed within a laser resonator with time.
For example, Wysocki et al. developed a laser capable of sweeping about 15 nm wavelength range at a rate of a few hundred Hz. The wavelength-swept laser used an erbium doped fiber as the gain medium and an acousto-optic filter as the wavelength sweep element(Reference: Optics Letters, Vol. 15, P879, 1990). Such a wavelength-swept laser together with a wavelength tunable laser is used in wide applications.
The wavelength-swept laser has been used in low interferometric distributed sensors, frequency range distributed sensors and fiber grating array sensors. It can also be used to analyze the wavelength characteristics of an optical device in a short time. Applications in the spectroscopy, optical communication and the like are motivated by the expectation that wavelength-swept lasers will take place of conventional wavelength tunable lasers, but examples of these applications have not been reported.
On the other hand, conventional wavelength-swept lasers have been operated with continuous wave, not with mode-locked wave. To obtain pulse type output, well-known active or passive mode-locking technology is generally required. In the case of the wavelength-swept laser, new mode-locking technology can be developed since the center wavelength of a filter placed within a laser resonator is continuously varied. However, no wavelength-swept laser has been reported that adopts such new mode-locking technology.
In general, a resonator mode of a laser is determined by the condition that the round-trip phase delay of laser light be some integral multiple of 2 &pgr;. Therefore, the frequency of m-th resonator mode has a value of f
m
=mc/L, where c is the velocity of light and L is the round-trip optical path of the resonator. If only single resonator mode oscillates in a laser, single frequency laser light with extremely narrow line-width is obtained. The output power of the laser light has a constant value with respect to time. If several resonator modes simultaneously oscillate, the laser output characteristics can be divided into two different states according to the amplitude and phase of the respective modes. First, if the amplitudes and phases of the modes have arbitrary relations or vary with time, continuous wave output is obtained. In this case, the power of the continuous wave output varies randomly with time due to so-called “mode partition noise”. Second, if both the amplitudes and phase differences of the modes are constant, short pulsed output is obtained by the coherent mode interference. This state is called “mode-locking”.
There are two techniques to achieve the mode-locking. In the case of active mode-locking, modulation frequency is adjusted equal to some integral multiple of the intermode spacing of longitudinal resonator modes using amplitude modulator, phase modulator, frequency shifter or the like. In the case of passive mode-locking, a saturable absorber, its equivalent optical device or resonator configuration is used.
However, an expensive modulator or an additional saturable absorber is required for the conventional mode-locking techniques.
DISCLOSURE OF INVENTION
It is therefore an object of the present invention to provide a wavelength-swept pulsed laser capable of obtaining short laser pulses without using an expensive modulator or an additional saturable absorber and to provide a method of generating such laser pulses.
It is another object of the present invention to provide a wavelength-swept laser capable of obtaining continuous wave output and to provide a method of generating such a laser output.
In order to accomplish the aforementioned object, the present invention provides a laser, comprising: a resonator having an optical path including therein an optical gain medium capable of amplifying light over wide wavelength band, a wavelength tunable filter with minimum loss center wavelength range, and a non-linear medium with light intensity dependent refractive index; an optical pump means for the population inversion of the optical gain medium; and a filter modulation signal generating means for continuously varying the minimum loss center wavelength range of the wavelength tunable filter with time; wherein the laser output is short mode-locked pulse type and its center wavelength varies continuously with time.
In the invention, the optical gain medium can be any one selected from a rare earth ion doped single mode optical fiber, a rare earth ion doped single mode planar waveguide, a titanium doped sapphire crystal or a Nd—YVO
4
crystal.
Moreover, a semiconductor amplifier may be used as the optical gain medium. In this case, the current supplied by a current generator pumps the semiconductor amplifier. Preferably, the current intensity is modulated at the intermode spacing of longitudinal resonator modes or some integral multiple of the intermode spacing to modulate the gain of the semiconductor amplifier, thereby helping the generation of the mode-locked optical pulses as well as adjusting the timing for the pulse generation.
In the invention, the wavelength tunable filter can be any one selected from the group consisting of an acousto-optic wavelength tunable filter, a Fabry-Perot interferometric wavelength tunable filter and a reflective refraction grating with varying reflective center wavelength depending upon rotation.
The wavelength tunable filter may include a beam deflection means and an optical device capable of producing low optical loss only within determined frequency range when the light transmitted or reflected depending on the controlled beam direction is coupled to the resonator. In this case, the beam deflection means can be an acousto-optic modulator that controls the beam direction according to the frequency of the acoustic wave. Otherwise, the beam deflection means may be a multiple phased array that controls the beam direction according to the phase differences of the respective light beams when light beams divided into several optical paths recombine together.
On the other hand, the non-linear medium may preferably include a length of single mode optical fiber or semiconductor material to enhance self-phase modulation effect and to act as a saturable absorber, thereby the non-linear medium helps the generation of mode-locked optical pulses.
However, the gain medium can act as a non-linear medium when the gain medium is a rare earth ion doped optical fiber having high non-linear coefficient or a titanium doped sapphire crystal. In this case, additional non-linear medium is not needed.
The resonator preferably includes an optical amplitude modulator for helping the generation of mode-locking as well as for adjusting the optical pulse generation timing, and a modulation signal generator for supplying alternating electrical signal to the optical amplitude modulator, the frequency of the electrical signal being equal to the intermode spacing of longitudinal resonator modes or some integral multiple of the spacing.
The optical amplitude modulator may be replaced with an optical phase modulator.
In order to accompli
Kim Byoung Yoon
Yun Seok Hyun
Korea Advanced Institute of Science and Technology
Marger & Johnson & McCollom, P.C.
Rodriguez Armando
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