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

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.

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

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.

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.

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)

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

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

Block Diagram of Internal Interfaces

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.

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