Optical: systems and elements – Collimating of light beam
Reexamination Certificate
2003-06-24
2004-07-27
Mai, Huy (Department: 2873)
Optical: systems and elements
Collimating of light beam
C359S710000, C385S015000, C385S033000
Reexamination Certificate
active
06768593
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
Not applicable.
BACKGROUND
1. Field of Invention
This invention relates to fiber-coupled laser diodes, specifically to fiber-coupled laser diodes that have both high coupling-efficiency and low feedback-noise.
2. Description of Prior Art
A fiber-coupled laser diode uses a fiber to transmit the laser diode light. Since the light emitted from the laser diode has already been launched into a fiber, a fiber-coupled laser diode can be readily used in optical-fiber networks for communications and sensing. In addition, a fiber-coupled laser diode is a flexible coherent light source, which can also be used in many optical devices, such as biomedical equipment, optical disk systems, laser printers, and others. A good fiber-coupled laser diode must provide both high coupling-efficiency to maximize the optical power output from the fiber and low feedback-noise to eliminate frequency and intensity fluctuations in the output light.
Coupling Efficiency
In order to obtain high coupling-efficiency between a laser diode and a single-mode fiber, i.e., to maximize the transmitted optical power through the fiber, the laser diode light distribution must match the node of the fiber. (See for example, M. Saruwatari and K. Nawata, “semiconductor laser to single-mode fiber coupler,” Applied Optics, Vol. 18, 1847-1856 (1979); M. Cote and R. R. Shannon, “Optimization of waveguide coupling lenses with optical design software,” Applied Optics, Vol. 35, 6179-6185 (1996)) The mode of a single-mode fiber has a circular Gaussian distribution. If the light distribution does not match the mode of the fiber, only part of the light enters and propagates through the fiber. Consequently, the output optical power from the fiber is low. On the other hand, if the light distribution matches the mode of fiber, the entire light will enter and propagate through the fiber. Thus, the coupling efficiency is nearly 100%. In other words, almost all optical power emitted from the laser diode is transmitted through the fiber.
The light emitted from a laser diode has an elliptical Gaussian distribution and astigmatic aberration. Thus, the emitted light must be corrected to a circular Gaussian beam, which must also be free from astigmatic aberration. A method for correcting the laser diode light using a microlens is described in J. J. Snyder, “Cylindrical micro-optics,” Proceedings of SPIE, Vol. 1992, 235-246 (1993); S. Jutamulia, “Correction of laser diode beam using microlens optics,” Optical Memory and Neural Networks, Vol. 10, 113-116 (2001); and U.S. Pat. No. 5,181,224 to Snyder (1993).
After the laser diode light is corrected to be free from astigmatic aberration, and to have a circular Gaussian distribution, the laser diode light is focused using an imaging lens onto the entrance end of the single-mode fiber. The beam-waist of the corrected Gaussian beam can be varied to match the mode of fiber by adjusting the distance from the microlens to the imaging lens, and the distance from the imaging lens to the fiber.
A prior-art fiber-coupled laser diode using a microlens is schematically shown in FIG.
1
. The astigmatic and elliptical beam emitted by a laser diode
20
is corrected by a microlens
22
to an astigmatic-aberration-free, circular Gaussian beam. The corrected beam is then focused by an imaging lens
24
to enter a single-mode fiber
26
.
Although high coupling-efficiency can be expected since the light distribution matches the mode of fiber, it has a severe drawback, i.e., high feedback-noise, as will be discussed in the following section. The feedback noise will generate frequency and intensity fluctuations in the output light. Thus, it will seriously limit the usefulness of the fiber-coupled laser diode.
Feedback Noise
A laser diode is a light-emitting device based on a light amplification effect. Light is amplified when it passes inside a laser diode. Similar to other lasers, a laser diode has a resonator. The resonator has a frequency response, while the light amplification effect has another frequency response. The overlapping area of two frequency responses will determine the lasing frequency (i.e., the frequency of the laser light). If no light from outside the resonator enters the laser diode, the light emitted by the laser diode will have a stable intensity and frequency. However, if light from outside the resonator enters the laser diode, the outside light will also be amplified and interfere with the light generated inside the resonator, resulting in frequency and intensity fluctuations in the emitted light. The outside light includes the light emitted from the laser diode and then reflected back into the laser diode. The reflection of a fraction larger than 10
−6
of the emitted optical power is sufficient to generate the intensity and frequency fluctuations. (W. Bludau and R. H. Rossberg, “Low-loss laser-to-fiber coupling with negligible optical feedback,” Journal of Lightwave Technology, Vol. LT-3, 294-302 (1985))
The prior-art fiber-coupled laser diode shown in
FIG. 1
suffers from severe intensity and frequency fluctuations caused by two feedback-noise sources: (1) reflection from microlens
22
, because it is very close (about 30 &mgr;M) to laser diode
20
, and (2) reflection from the end of fiber
26
, because the light reflected from the end of fiber
26
is focused back by lens
24
and microlens
22
to laser diode
20
.
Antireflection (AR) coatings reduce the reflection to only about 10
−3
of the incoming power which is not sufficient to suppress the feedback-noise. To reduce the reflection from the end of fiber
26
to enter the laser diode, an angled fiber can be used (i.e., the end of fiber is polished to form an angle that is not perpendicular to the fiber axis). However, this will substantially reduce the coupling-efficiency, since the incoming light will be bent at the entrance end of angled fiber when it enters the angled fiber. The mode matching condition is no longer preserved when the incoming light is bent. The reflection from microlens
22
alone, which is AR coated, is sufficient to generate intensity and frequency fluctuations These unwanted fluctuations limit the application of the apparatus. For example, the intensity fluctuation precludes its use in optical disk systems and the frequency fluctuation precludes its use in laser diode pumped solid-state lasers.
An attempt to make a fiber-coupled laser diode having both high coupling efficiency and low feedback-noise has been made using a microlens, which is directly fused to the fiber. (W. Bludau and R. H. Rossberg, “Low-loss laser-to-fiber coupling with negligible optical feedback,” Journal of Lightwave Technology, Vol. LT-3, 294-302 (1985)) Although the feedback-noise can be suppressed to 10
−7
of the emitted light power by placing the microlens exceeding 100 &mgr;m away from the laser diode and directly fusing the microlens to the fiber, it fails to obtain high coupling-efficiency. The coupling efficiency is only between 40% and 70%. The fused microlens is fabricated based on an empirical method, instead of precise design and production. Thus, the microlens suffers from severe aberrations, and the coupling efficiency cannot be high. In addition, it cannot be produced in volume.
To summarize, the prior-art fiber-coupled laser diode shown in
FIG. 1
is capable of providing a coupling efficiency as high as >90%. (S. Jutamulia, “Optical communications: technology and economy,” Proceeding of the 2002 International Conference on Opto-Electronics and Laser Applications, A-07-A-10 (2002) ISBN: 979-8575-03-2) However, it suffers from severe intensity and frequency fluctuations generated by feedback noise. On the other hand, a prior-art fiber-coupled laser diode using a microlens filed to the fiber may suppress feedback noise, but its coupling efficiency is low (<70%).
Objects and Advantages
Accordingly, several objects and advantages of the present invention are:
(1) to provide an improved fiber-coupled laser diode;
(2) to provide a fiber-coupled laser diode with high
LandOfFree
Fiber-coupled laser diode having high coupling-efficiency... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Fiber-coupled laser diode having high coupling-efficiency..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fiber-coupled laser diode having high coupling-efficiency... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3188160