Sub-mount having photodiode and light-emitting element module

A light-emitting element module, comprising:

• a laser diode light-emitting element that emits light having an emission wavelength which changes as temperature changes within a range of −10° C. and 90° C.,
• a sub-mount comprising a photodiode having a first conductivity-type layer arranged in a surface portion of the sub-mount in a photodiode region and configured to form a light-receiving surface, and having a second conductivity-type region arranged below the first conductivity-type layer, the photodiode being configured to receive at the light-receiving surface a light emitted from the light-emitting element and to convert the light into a photocurrent,
• wherein the light-emitting element is fixed on a surface of the sub-mount in a light-emitting element region adjacent to the photodiode region,
• wherein the photodiode additionally has a depletion layer generated from the first conductivity-type layer, the depletion layer having a thickness in the range 20 μm to 45 μm, and
• wherein a peak light-receiving wavelength at which the photocurrent of the photodiode becomes its maximum value is more than or equal to a minimum emission wavelength of the light-emitting element and less than or equal to a maximum emission wavelength of the light-emitting element.