Refering to Fig 1 and the discussion starting on page 2 on the link:
patents.ibm.com
The core of the invention is the TRANS LASER, the transmission laser, whose job it is to generate the modulated photonic beam which will be sent out over the fiber cable. It is referred to as section 12, or just 12. There are three supporting systems which condition the laser 12 to be optimally operating according to required outputs. Two of these supporting systems are optoelectronic feedback loops 13, and 14, whose job is to control the temperature of 12 according to a wide range of demands placed on it within a narrow range of acceptable operating efficiencies. Instabilities that arise in the transmission laser cavity and stimulated emission mass have to be quickly dampened so these systems must be extremely and discretely responsive. In addition section 14 feedback loop serves other systems by providing beat frequency clock input. Perhaps the most controversial portion of the device is section 16 modulator, although I don't yet understand why it is so mysterious.
DEF: Mode
1. The characteristic of the propagation of light through a waveguide that can be designated by a radiation pattern in a plane transverse to the direction of travel.
2. The state of an oscillating system such as a laser that corresponds to a particular field pattern and one of the possible resonant frequencies of the system.
DEF: Cavity
In a laser, the optical resonator formed by two coaxial mirrors, one totally and one partially reflective, positioned so that laser oscillations occur.
DEF: Active Layer
That layer in a semiconductor injection laser or light-emitting diode that provides optical gain.
DEF: Grating
A framework or latticework having an even arrangement of rods, or any other long narrow objects with interstices between them, used to disperse light or other radiation by interference between wave trains from the interstices. The ability of a grating to separate wavelengths, (chromatic resolving power) is expressed as being equal to the number of lines in the grating.
DEF: Gain
Also known as amplification. 1. The increase in a signal that is transmitted from one point to another through an amplifier.
DEF: Distributed Feedback Laser
A laser system in which feedback is used to make certain modes in the resonator oscillate more strongly than others. In semiconductor lasers, a periodic corrugation in the active layer replaces the cleaved end mirrors, and the grating spacing is chosen to distribute the feedback in both directions, creating a condition that can approach single-mode oscillation. Distributed feedback lasers can be fashioned on a monolithic device and show promise for integrated optical circuits.
Palmer's discussion follows:
FIG. I is a block-diagram of a laser modulation system 10 generally in accordance with an embodiment of the invention. Included within the modulation system 10 is a transmitting laser 12, a first laser stabilization feedback section(circuit) 13, a second laser stabilization feedback section (circuit) 14 and a laser modulation section 16.
The transmitting laser 12 may be any distributed feedback(DFB) laser (13,000 Angstrom units wavelength) compatible with an appropriate temperature control. The junction current of the laser 12 is provided by a current controlled source (not shown) supplying a rated current with no more that O.1% ripple.
I am using Angstroms where sometimes he uses um, micro millimeters. 10^(-6) meters. There are 10^(-10) meters/Angstrom unit.
The current ripple(amplitude deviation or transient) has to be extremely small because the transmission laser needs all superfluous inputs minimized and is sensitive to variations.
Temperature control of the laser 12 is accomplished using an active heat control device 18 and balancing heat source 20. The active heat control device 18 can be a temperature control device located within a mounting surface (e.g. a heat sink) of the. laser 12. The device 18 may be implemented using a thermocouple sensor and controller coupled to any thermally active temperature control device (e.g., a Peltier effect thermoelectric heater/cooler). The operating temperature of the heat control 18 of the laser 12 may be held to an appropriate set point temperature (e.g. 32' F.) with an appropriate limitation on temperature variation (e.g., no more than 0.1 F).
This paragraph make it apparent that the heat stability issue is paramount.
The balancing heat source 20 is directed to stabilization of cavity dimensions by temperature control and may be implemented using an appropriately sized nichrome wire wrapped around the outside of the cavity of the laser 12 and surrounded by a thermally conductive, electrically non-conductive material (e.g., soreison, etc). The heat source 20 may be used to provide an appropriately stable cavity temperature (e.g., 32.l F +- .001 F) to restrict cavity mode hopping of an output of the DFB laser 12.
The cavity dimensions are the effective heat volume of the stimulated emission mass. This volume controls the emission efficiency through the two components of lasing, heat transparency and electron to photon orbital capture and release. When the atoms of the laser crystal are vibrating from heat of emission, if the temperature of the heat is too high, the resistance to transmission rises. If lower, heat transparency increases and stable photon flux rises. In the background Palmer says, "Static variations in the inside cavity dimensions may cause the cavity to inherently resonate at a number of frequencies. Variations in the junction current may cause a center frequency to shift (i.e., hop) from one resonant regime to another. Changes in cavity dimension caused by temperature may have the same effect." At some lower point, a critical value or threshold is reached such that lasing stops. So within the desired range of temperature and only there will the laser function properly. This is so critical that fine tuning accomplished by winding nichrome wire around the laser cavity cylinder may be necessary. It seems to depend on economic constraints on the quality of the laser crystal.
Next Palmer segues into section 14 feedback loop:
The paradigm laser 40 may be a low power diode laser operating with a junction current fixed to within 0.1% and operating at 13,000 Angstrom units wavelength. The paradigm laser 40 may be calibrated using an appropriate instrument standard (e.g., a Zeiss DK-2 Spectrophotometer using a quartz-iodine lamp that is NBS traceable) to a known energy level in each wavelength. The paradigm laser 40 may also be stabilized using a temperature controlled heat sink and an active temperature controller similar to that used by the transmitting laser 12.
DEF: paradigm laser (generic)
Paradigm Lasers Inc develops and manufactures diode pumped solid-state laser systems using their patented radial diode array packaging technology. Their radial packaging is ideally suited for constructing high brightness/high power diode pumped solid-state lasers
Notice that he brings up stabilizing and precisely calibrating this raw input component device.It's output must be known and clean.
We have been dancing around the issue, so it is time to get to the meat of the matter:
The fundamental problem associated with the stability of the transmitting laser 12 and paradigm laser has been determined to be control of the resonant modes operating within the cavity. Control of the resonant modes, in turn, is highly dependent upon the dimension of the laser cavity. The temperature of the cavity has been determined to be a significant factor in the cavity dimension and laser stability. Further, where attempts are made to control the cavity temperature, the cavity temperature often overshoots a set point due to the thermal lag (and thermal mass) associated with each laser 12, 40. The solution to the problem in fact has been found to lie in control of the cavity temperature by modelling the laser cavity as a transient thermodynamic system. Using embedded thermistors and a summing operation amplifier, it has been found that the active temperature controller 18 can be adapted to follow the transient temperature using techniques previously described by the inventor.(for example, Palmer, J.R., Transient Heat Transfer in Flat Plates, Vol. II Constant Temperature, Pro Sc Publications, San Diego, CA (1995)).
DEF: Resonance Radiation
That radiation emitted by an atom or molecule that has the same frequency as that of an incident particle, e.g., a photon. It generally involves a transition to the lowest energy level of the atom or molecule.
DEF: Resonator
A volume, bounded at least in part by highly reflecting surfaces, in which light of particularly discrete frequencies can set up standing wave modes of low loss. Often, in laser work, the resonator contains two facing mirrors that may either be flat (Fabry-Perot resonator) or have some spherical curvature.
DEF: Thermistor
A solid-state semiconducting structure that changes electrical resistance with temperature. Materially, some kind of ceramic composition is used.
DEF: Amplifier
A device that enlarges and strengthens a signal's output without significantly distorting its original wave shape. |
