1. Laser(s) for Keck Observatory’s Next Generation AO (NGAO) System
Jason Chin, Chris Neyman, Viswa Velur
November 02, 2007

2. Agenda Brief NGAO status & goals to provide context
Provide an outline of the required laser to possible laser vendors and acquire feedback.
Major considerations for the laser.
Determine feasibility of existing vendor designs and determine how existing lasers can be redesigned to meet the needs of the NGAO laser.

3. NGAO Keck’s Next Generation Adaptive Optics System
In support of Keck’s Strategic Goals
Leadership in high angular resolution astronomy
State of the art instrumentation
Complementarity with ELTs
New Capabilities
Dramatically improved near-IR performance
Increased sky coverage
AO correction at red wavelengths
Instrumentation to facilitate the range of science programs

4. NGAO Roadmap Currently in System Design Phase
Development of System Architectures
Development of Requirements
Provide Conceptual Designs
System Design Review in late March of 2008
Procurement of laser(s) in 2010
Installation of laser(s) in 2012
Science verification in 2013
Motivation for discussion today
Guidance in moving forward with NGAO laser facility conceptual design.
Provide conceptual design for feasibility and costing estimates.

5. Technical laser requirements
Generation of 9 laser guide stars (LGS).
Power
Each LGS with an expected return of 1200 photons/sec/cm2 at the telescope entrance.
Assuming a 0.75 efficiency factor, estimate laser output at 1600 photons/sec/cm2. This may vary depending on laser architecture.
Based on current SOR figures of 100 photons/sec/W/cm2, this would equate to 16W for each LGS.
Based on the current LMCTI figure of 33 photons/sec/W/cm2, this would equate to 48W for each LGS.
Stability
Long term, power < 10% fluctuations over the 12 hour observing period.
Short term, power < 5% fluctuations.
Bandwidth and Frequency
< TBD GHz.
Stability of 50 MHz.
Tunability in 20 MHz steps, range of 1.5 GHz, speed (TBD)
Measurement of Rayleigh background, Peaking power.
Angular Stability and Transverse Stability
Desired Polarization: 100:1 circular or linear polarized.
Beam quality: M2 of 1.2 or better
Diagnostics
Power, beam quality measurement, spectral bandwidth, M2.

6. How to make NGAO’s decision?
Criteria and priorities in guiding the architecture selection process
Costs
per beacon for a given sodium return & spot size
to integrate laser(s) at telescope
to operate & maintain laser(s)
Impact on facility/operations to integrate laser(s)
Laser size & location (changing gravity vector acceptable?)
Compatibility with software architecture
Safety
Impact on facility/operations to operate & maintain laser(s)
System complexity & reliability (How expert do staff need to be?)
Failure modes (slow or catastrophic?)
Future vendor support for laser
Performance issues
Beam quality (spot size limitations imposed by laser)
Compatibility with single mode fiber transport
SNR (fratricide and background may be less for pulsed lasers)
How well do we understand the laser format and vouch for the Na return (important when considering new laser formats)
Upgradeability.
Additional power
How well the laser technology is adaptable to techniques (like 2 color Na pumping, multi-color LGS to get rid of the tilt indeterminacy)

7. Potential Laser Locations
Right Nasmyth Deck; stable gravity vector; higher beam transport losses. On or below platform.
Elevation Ring; moving gravity vector; lower beam transport losses.
Secondary Module; moving gravity vector; limited volume; minimal beam transport losses

8. Integration Considerations
Size requirements
Minimizing laser footprint. Planned K1 laser fits within an enclosure on the Right Nasmyth Platform. K2 laser amplifiers fit in enclosure on elevation ring.
Gravity vector effects. Stable gravity for K1 laser and varying for K2 laser.
Laser enclosure(s)
Clean room requirements
Temperature stability requirements
Power requirement
Cooling requirements
Software interface to observatory (EPICS distributed system control)
Remote control of lasers from headquarters.
Safety requirements

9. Block Diagram of Internal Interfaces

10. Feasibility and Issues
Laser
Requirements compliance
laser return, power, beam quality, etc..
Can laser operate with changing gravity vector?
What is the expected size of the laser to produce the necessary returns?
How many lasers?
Scalability; what step size in laser power per $$
upgrade system with more lasers or amplifiers? As funding allows or “all in” investment
Other laser issue notes
Magnetic enhancement at Hawaii lat, longitude. (Similar to Maui)
Nightly variations in Na density (Similar to Maui).
Are existing CW (SOR) & mode-locked CW (LMCTI) results in literature typical?
Can existing pulse formats be modified to further improve returns?
Is optical pumping with both D2a and D2b lasers useful at powers of 150/9= 16 watts per LGS.
Range gating of laser pulses to reduce overall power requirement and Rayleigh background.

11. Feasibility and Issues
Integration
Laser locations, what can be off the telescope, on Nasmyth, on elevation ring or else where? This can significantly affect laser throughput.
Infrastructure Requirements.
Power and cooling.
Clean room.
Software interface and control.
Operations, reliability and serviceability
Effort level to operate and service.
Operational cost for spares per year over 10 year lifetime.
Failure modes and downtime estimates.
Safety requirements and concerns