3.6.1 Methods of increasing the output power
In section 3.3 it was explained that two coupled processes control the out- put power of the phase conjugate oscillator: the diffraction efficiency of the grating and the gain seen by the phase conjugate beam.
These conditions needed to optimise these processes are unfortunately mutually incompatible as the variable attenuator needs to be set to its maximum transmission in one direction and to near its minimum in the other. What is needed is some nonlinear loss mechanism in the loop which will allow a low power beam to pass in one direction whilst attenuating a higher power beam travelling in the other direction.
One solution to this problem is a non-reciprocal transmission device using a ”leaky” Faraday isolator. This is unsuitable for use in a monolithic sys- tem but will be discussed further and its implementation described in the next chapter.
3.6.1.1 Controlling the transmission with an aperture
Another possible solution to this problem is the insertion of an aperture into the loop.
When a finite-sized beam is apertured its power is reduced. If this atten- uated beam is then reflected by a phase conjugate mirror (PCM). The re- flected phase conjugate will pass back through the aperture unattenuated. By varying the diameter of the aperture the power of the writing beam can be controlled whilst maintaining 100% transmission in the opposite direction.
Unfortunately the aperture method of controlling the power ratio has sev- eral major disadvantages. If a real Gaussian beam is passed through an
PCM
PCM
Figure 3.18: Inserting an aperture into a beam which is then phase conju- gated.
aperture diffraction effects will act to distort the beam. This will result in a loss of information at the phase conjugate mirror and therefore a distorted reflection.
PCM
Figure 3.19: Diffractive effects from an aperture limit the ideal phase con- jugate behaviour.
Beam steerage due to the thermal load in the amplifier is another issue to be contended with (this will be covered more fully in chapter 5). When the amplifier is pumped the refractive index changes by an amount pro- portional to its temperature. In a side-pumped geometry this causes the beam’s path to deviate by an amount proportional to the pump power. The beam can easily become misaligned with respect to the aperture caus- ing the transmission to vary.
3.6.2 A monolithic resonator
The experiments performed in this chapter show that a (all be it unstable and multi frequency) self intersecting phase conjugator operating via gain gratings with minimal optics can be built. This has the potential for de- velopment into a monolithic phase conjugator of a similar design to photo refractive phase conjugators.
A possible schematic of a monolithic phase conjugator is shown in figure 3.20. Here a seed beam is launched into a side-pumped crystal and re-
flected off the pumped face. The beam is then reflected off a corner of the crystal such that it intersects with itself in the region of highest gain. The angle that the seed beam meets the reflective faces would be selected such that their reflectivities would optimise the transmission of the loop and therefore the diffraction efficiency of the gain gratings.
PC
Seed beam
Pump beam
Figure 3.20: A possible schematic for a monolithic phase conjugator.
In order to achieve phase conjugation in the CW regime very high gains are needed. The drawback with high gains is the possibility of power loss through parasitic lasing. The reflective surfaces needed to steer the beam greatly increase the risk of parasitics and multiple pass ASE. This whole topic will be discussed in more detail in chapter 6.
3.7 Conclusions
The oscillator generates a phase conjugate of its seed beam adapting to phase distortions inserted into its cavity. An output power of ∼1W is achieved with a slope efficiency of ∼5.5%. The output power and lon- gitudinal modal structure fluctuate rapidly. These fluctuations are due to the phase shift between the gain grating and its interference pattern. Sev- eral temporal modes are constantly competing for gain suppressing and supporting each other seemingly at random. This complex system results in the power fluctuations that we see in the output.
3.8 References
[1] J.M. Hendricks. Holographic laser resonators. PhD thesis, School of Physics, Faculty of Science, University of Southampton, 2002.
[2] Casix. Neodymium doped yttrium orthovanadate (Nd:YVO4) crystal,
2001. http:\\www.casix.com.
[3] O. Wittler, D. Udaiyan, G. J. Crofts, K. S. Syed, and M. J. Damzen. Characterization of a distortion-corrected Nd:YAG laser with a self- conjugating loop geometry. IEEE Journal of Quantum Electronics, 35(4):656–664, 1999.
The holographic resonator
4.1 Introduction
In this chapter the phase conjugate oscillator (described in the previous chapter) will be developed into a stable laser system. A non-reciprocal transmission element (NRTE) is inserted into the loop of the oscillator, which acts to improve the efficiency of the system and enable single fre- quency operation by removing the phase shift between the grating reading and writing beams. The oscillator will be built in both side loop and ring geometries, and it will then be modelled and characterised. A self-starting version of the oscillator will also be built and again modelled and charac- terised.