Ruby Maser

A ruby maser is a specific type of maser that uses a synthetic ruby crystal as the amplifying medium to generate coherent microwave radiation.

A maser, which stands for Microwave Amplification by Stimulated Emission of Radiation, works on the same principle as a laser but produces microwave photons instead of visible light photons.

key properties and features of a ruby maser

  • The amplifying medium is a rod of synthetic ruby, which is an aluminum oxide (Al2O3) crystal doped with trace amounts of chromium (Cr) ions. This gives the ruby its red color.
  • It uses a solid-state medium, making it relatively easy to work with compared to gas lasers.
  • The ruby rod is placed between two highly reflective mirrors, forming an optical cavity.
  • Energy is pumped into the ruby using a flash lamp, which excites the Cr ions into a higher energy level.
  • As the Cr ions decay to lower levels, they emit photons in the microwave frequency range.
  • Photons bouncing between the mirrors stimulate the emission of more identical photons, amplifying the microwaves through stimulated emission.
  • The coherently amplified microwaves are emitted through one partially reflective mirror as the output beam.
  • Ruby masers can generate pulsed or continuous coherent microwave radiation, typically in the X-band (8-12 GHz) frequency range.
  • Ruby masers were instrumental in early maser research but have been superseded by better masers using hydrogen, ammonia, etc. They are rarely used today compared to other maser types.


The RUBY MASER consists of a ruby crystal kept in the cavity. The cavity is enclosed by a jacket of liquid helium in order to observe the heat generated by the ruby crystal during operation.

For further cooling the jacket of liquid nitrogen is used at the outermost surface of the ruby maser. A pump input is provided to the cavity in order to excite the ruby crystal.

The input and output of microwave frequencies are given through the device known as a circulator. A permanent magnet is also used across the maser.

Ruby Maser Construction


When the pump input is provided to the cavity the electron in the ruby maser moves from the lower energy band to the higher energy band. The population inversion takes place which results in to fall back of the electrons from the high energy band to the lower energy band. During this time the electrons emit the photons which produces the microwave frequency. The field of this frequency excites the cavity. As a result, the oscillation takes place inside the cavity at the microwave length of frequency.

When the high-range radio frequency input signal is applied to the cavity through the circulator, It finds itself in the high level of the signal at the same frequency inside the cavity so the output of this signal is taken from the output port of the circulator in amplified form.

The input and output port between the circulator and cavity is the same. It is possible due to the reason that when the input is maximum at the input port. It is minimum at the cavity port because the distance between these two ports is λ/4. The same is the case for the output signal.

Applications of ruby masers

  • Atomic clocks

    One of the early and important uses of ruby masers was in atomic clocks starting in 1960. The precise microwave frequencies generated by ruby masers enabled accurate timekeeping needed for navigation systems, telecommunications, etc. Ammonia and hydrogen masers eventually replaced ruby masers in this role.

  • Amplifiers

    The coherent microwaves produced by ruby masers allowed them to be used as low-noise microwave amplifiers. They were used to amplify signals in communications systems and radar equipment. Later superseded by semiconductor-based amplifiers.

  • Spectroscopy

    The narrow frequency emission of ruby masers made them useful as frequency standards for spectroscopy to probe atomic energy levels in a wide range of samples.

  • Physics research

    Ruby masers were instrumental in the early research on maser physics, leading to the invention of the laser and the understanding of light/matter interactions. Important for studying stimulated emission.

  • Medical imaging

    Some early experiments used ruby masers as microwave sources for medical imaging techniques like cyclotron resonance imaging. But lacked imaging resolution.

  • Quantum optics

    Ruby masers enabled research into quantum optics and phenomena like the antibunching of photons when emitting single microwave photons.

  • High stability oscillators

    The precise, stable microwave frequencies enabled ruby masers to be used as low-noise oscillators for communication or radar systems that required exceptional frequency stability.

  • Microwave heating

    Early research into using maser beams for targeted microwave heating and welding of materials. But lacked power for practical use.

Ruby masers found early use as precision frequency sources until superseded by better alternatives, but were instrumental in the development of maser and laser technology.

Merits and Demerits of ruby masers:


  • Simple design - Ruby masers have a simple solid-state design using a synthetic ruby rod, making them easy to construct and maintain compared to gas lasers.
  • Coherent emission - Generate highly coherent and monochromatic microwave radiation, useful for applications like spectroscopy.
  • Precise frequency - Early ruby masers enabled very precise and stable frequency standards needed for atomic clocks.
  • High amplification - Can amplify microwaves to high powers, enabling use as low-noise microwave amplifiers.
  • Continuous or pulsed - Capable of operating in continuous wave or pulsed output modes.
  • Compact size - Ruby masers are relatively small devices, making them portable for applications like atomic clocks.


  • Limited tuning - The emitted microwave frequency cannot be easily tuned over a wide range. Restricted to fixed emission bands.
  • Low power - Do not produce very high power microwave output compared to other maser designs.
  • Superseded as clocks - Have been superseded by better masers using hydrogen, ammonia, etc. as frequency standards for atomic clocks.
  • Instability - Output stability and lifetime are more limited compared to other maser designs. Require frequent adjustment and recalibration.
  • Noisy - Later ruby masers tended to have higher noise than other microwave amplifiers.
  • Low efficiency - They have relatively low energy efficiency in converting pump energy into useful microwave output.

Ruby masers were instrumental historically but had limitations in tunability, power, and efficiency compared to later maser developments. Their roles became much more specialized and niche over time.