The link:
patents.ibm.com
As mentioned above, the paradigm laser 40 is a low power diode laser operating at 13,000 Angstroms wavelength. Prior to entry into the paradigm monochrometer 38, the laser beam from the paradigm laser 40 is passed through a 300 Angstrom line filter as a rough control before entry into the monochrometer 38.
Within the monochrometer 38, the beam passes through the expander-collimator 52, onto the grating 54, and is reflected from the parabolic mirror 56, as described above and is passed on by the rotating mirrors 62 and grating 66 of the opto-electronic clock, 34. After passing through the grating 66, the beam strikes a InGaAs detector 70 matched to another InGaAs detector 68 detector on the other side of the optical phase lock loop 36. As mentioned above, the paradigm laser 40 is calibrated using the Zeiss DK-2 Spectrophotometer to precisely determine the optical output of the laser 40.
With the transmitter laser 12 slaved to the paradigm laser 40 through the optical phase lock loop 36, the variable controlled output 54 will feed back to the cavity temperature control 20 of the transmitter laser 12 as small increments of voltage control on the heating element surrounding the optical cavity of the transmitting laser 12. In this manner, it has been found that temperature (and length) of the cavity can be more precisely controlled. The feedback signal 54 of the second feedback circuit 14, in fact, has been found to provide a granularity of control 53 times better than the first feedback circuit 13
That should answer some questions about the purpose of section 14.
Turning now to the opto-electronic clock 34, it should be noted, first, that the opto-electronic clock 34 functions as a portion of the second feedback circuit 14 and also as a clock source 45 for modulating information signals onto the laser beam. For purposes of simplicity, FIG. 3 will be used to explain both functions.
The laser transmission system 10 of FIG. 1 is provided for the modulation and transmission of up to 1,000 television channels. Since each television channel (including guard bands) requires 6.2 MHZ, the modulation capabilities of the system 10 of the invention would be expected to be at least 6.2 Ghz.
Further, to modulate a channel onto the laser beam, a clock signal is required every 6.1 MHZ throughout the transmission spectrum. One of the features of the clock 34 is a practical method of providing a multitude of such incremental frequencies.
And therefore it must be synchronized to laser 12, laser 40, and crystal opto-modulator. All three must be co-synched.
The essence of the clock 34 is two counter-rotating disks 72, 74. The two disks 72, 74 may be driven by variable speed motors (e.g., Dremel motors). The variable speed capability of the disks 72, 74 is such that the speed may be continuously varied from a low of 83.33 revolutions per second(rps) to 25.3 x 10^4 rps. Under an embodiment, the speed of the inner disk, 72 is chosen to be 500 rps. The two disks 72, 74 rotate in opposite directions.
A number of mirrors 60, 62 are mounted to the inner mirror disk 72. More specifically, sixteen mirrors 60, 62 are mounted at a 4-S' angle to a rotational axis of the disk 72 around the periphery of the disk 72 at a radius of 63.5 mm.
The mirrors 60, 62 are mounted such that a light beam traveling parallel to the axis of rotation of the disk 72 would be reflected radially through a rotating transmissive grating 64, 66,on the second disk 74 at a predetermined angle of the disks 72, 74 to strike a set of stationary detectors 68, 70.
The second disk 74 rotates at a speed of 15.27887 x 10^3 rps. The second disk may be equipped with an annular ring 76 of a radius of 97.0209 mm, fitted with a rotating transmissive grating 64, 66.
The total cycles per second produced by the opto-electronic clock 34 may be determined by the equation;
Vo = a*b*(Nm*x + Y),
were
a = grating lines/mm,
b = circumference of the second disk,
x = rps of the mirror disk-,
Y = rps of the second disk and
Nm = number of mirrors on the mirror disk.
As should be apparent from the equation, the frequency of the signal detected by the detectors 68, 70 can be adjusted 55 by changing the gratings, the number of mirrors, the radius of the mirrors, or the radius of the grating. Having passed through the grating 64, 66, the laser beam strikes the detectors 68, 70 providing an electrical signal that is conditioned and amplified to establish a particular tuning frequency.
For purposes, of the second feedback circuit 14 alone, the speed of the counter-rotating disks 72, 74 is not considered critical. As a consequence, the disks 72, 74 may be operated at a speed convenient for generation of reference frequencies. Of more importance for purposes of the second feedback circuit 14 is the ability of the, disks 72, 74 to chop a light beam from both the transmitting laser 12 and from the paradigm laser 40 into light pulses that may be compared as to phase and intensity.
FIG. 4 is a block diagram of the phase lock loop circuit 36. As shown, an output of the sensors 68, 70 is shaped within an op amp 102,104 and Schmidt trigger 106, 108. The shaped pulses are passed through a loop filter 130, 132 before comparison in. a control logic section 128.
DEF: Schmidt Trigger
Oscilloscope electronic circuit that produces an output pulse whose pulse width is determined by the time that the output voltage exceeds a specified level.
From the detectors 68, 70, the level of the signal requires amplification to raise the signal to a sufficient level to trigger the Schmidt trigger 106, 108. The signal from the detector of the paradigm laser 40 uses a non-inverting op amp 104. The signal from the detector 68 of the transmitting laser 12 may use an inverting op amp 102. It should be noted that it is not all that important that the input signal from the detectors 68, 70 match, but only the square wave of the Schmidt triggers 106, 108 substantially match.
The loop filters 130, 132 provide a means of averaging an output of the phase lock loop 36. A signal from the loop filter 130, 132 is compared with an output of the Schmidt trigger 106, 108 in control logic 128 (e.g., an XOR gate) to identify and reinforce coincidence of phase and amplitude relationships between the transmitting laser 12 and paradigm laser 40. The output of the control logic 128 is then scaled and provided as a control output of the second feedback circuit 14 through the heater control logic 20.
The PLL keeps the transmission laser in synch with the beat laser 40 co-synchronized by the optoclock.
The mechanical optoclock can be replaced by an equivalent with no moving parts. Apparently SR had done this in the next embodiment. Later models had other refinements and one of later models was demoed in New York, 11/98. The patent only was for a 6 gig prototype and when it was spinning and whirring, it made the devil's own noise. |
