Varifocal display with wavelength tuning

1. A display device, comprising:

a tunable light source operable in different states including a first state and a second state, wherein:

the tunable light source provides:

first light having a first wavelength while the tunable light source is in the first state;

second light having a second wavelength distinct from the first wavelength while the tunable light source is in the second state;

third light having a third wavelength distinct from the first wavelength and the second wavelength while the tunable light source is in the first state; and

fourth light having a fourth wavelength distinct from the first wavelength, the second wavelength, and the third wavelength while the tunable light source is in the second state;

the first wavelength and the second wavelength correspond to a first color band; and

the third wavelength and the fourth wavelength correspond to a second color band distinct from the first color band; and

a diffractive optical element positioned to:

receive the first light and redirect the first light, wherein the diffractive optical element has a first focal length for the first light;

receive the second light and redirect the second light, wherein the diffractive optical element has a second focal length, distinct from the first focal length, for the second light;

receive the third light and redirect the third light, wherein the diffractive optical element has a third focal length for the third light; and

receive the fourth light and redirect the fourth light, wherein the diffractive optical element has a fourth focal length, distinct from the third focal length, for the fourth light, wherein:

the third focal length is distinct from the first focal length; and

the display device further includes a wavelength-dependent optical element positioned to receive and modify at least one of the first light and the third light so that both the first light and the third light have a common virtual display plane.

2. The display device of claim 1 , wherein:

the tunable light source forgoes providing the second light having the second wavelength while the tunable light source is in the first state; and

the tunable light source forgoes providing the first light having the first wavelength while the tunable light source is in the second state.

3. The display device of claim 1 , wherein:

while the tunable light source is in the first state, a user of the display device perceives an image rendered by the first light at a first distance; and

while the tunable light source is in the second state, the user of the display device perceives an image rendered by the second light at a second distance distinct from the first distance.

4. The display device of claim 1 , wherein:

a difference between the first wavelength and the second wavelength is at least 20 nanometers; and

a difference between the first focal length and the second focal length is at least one millimeter.

5. The display device of claim 1 , wherein:

the tunable light source provides light having a first wavelength range including the first wavelength while the tunable light source is in the first state;

the tunable light source provides light having a second wavelength range including the second wavelength while the tunable light source is in the second state; and

the first wavelength range has a first bandwidth and the second wavelength range has a second bandwidth, the first bandwidth and the second bandwidth being less than two nanometers.

6. The display device of claim 1 , wherein:

the second wavelength is longer than the first wavelength, and the second focal length is shorter than the first focal length.

7. The display device of claim 1 , wherein:

the tunable light source switches from the first state to the second state sequentially.

8. The display device of claim 1 , wherein:

the tunable light source in the first state provides the first light and the third light concurrently at a first time and the tunable light source in the second state provides the second light and the fourth light concurrently at a second time distinct from the first time.

9. The display device of claim 8 , wherein:

the tunable light source forgoes providing any of the second light and the fourth light while the tunable light source provides the first light and the third light; and

the tunable light source forgoes providing any of the first light and the third light while the tunable light source provides the second light and the fourth light.

10. The display device of claim 1 , wherein:

the tunable light source further provides:

fifth light having a fifth wavelength while the tunable light source is in the first state; and

sixth light having a sixth wavelength while the tunable light source is in the second state;

the fifth wavelength and the sixth wavelength correspond to a third color band distinct from the first color band and the second color band; and

the diffractive optical element is positioned to:

receive the fifth light and redirect the fifth light, wherein the diffractive optical element has a fifth focal length for the fifth light; and

receive the sixth light and redirect the sixth light, wherein the diffractive optical element has a sixth focal length, distinct from the fifth focal length, for the sixth light.

11. The display device of claim 10 , wherein:

the first color band corresponds to red, the second color band corresponds to green, and the third color band corresponds to blue.

12. The display device of claim 1 , wherein:

the wavelength-dependent optical element includes a Pancharatnam-Berry phase (PBP) lens or a wavelength-multiplexing holographic optical element.

13. The display device of claim 1 , wherein:

the fourth focal length is distinct from the second focal length; and

the wavelength-dependent optical element is positioned to:

receive and modify at least one of the second light and the fourth light so that

both the second light and the fourth light have a second virtual display plane, distinct from the common virtual display plane.

14. The display device of claim 1 , wherein:

the tunable light source also provides:

third light having a third wavelength distinct from the first wavelength and the second wavelength while the tunable light source is in a third state distinct from the first state and the second state; and

fourth light having a fourth wavelength distinct from the first wavelength, the second wavelength, and the third wavelength while the tunable light source is in a fourth state distinct from the first state, the second state, and the third state;

the third wavelength and the fourth wavelength correspond to a second color band distinct from the first color band; and

the diffractive optical element is positioned to:

receive the third light and redirect the third light, wherein the diffractive optical element has a third focal length for the third light; and

receive the fourth light and redirect the fourth light, wherein the diffractive optical element has a fourth focal length, distinct from the third focal length, for the fourth light.

15. The display device of claim 1 , further comprising:

a spatial light modulator positioned to:

receive the first light at a first time and the second light at a second time distinct from the first time; and

provide modulated light toward the diffractive optical element.

16. A set of one or more controllers configured for use with the display device of claim 1 , the set of one or more controllers comprising:

one or more processors; and

memory storing one or more programs for execution by the one or more processors, the one or more programs including instructions for:

placing the tunable light source in the first state for providing the first light having the first wavelength;

placing the tunable light source in the second state for providing the second light having the second wavelength distinct from the first wavelength; and

adjusting an output power of the tunable light source so that a perceived intensity of the second light by a user of the display device is substantially the same as a perceived intensity of the first light by the user.

17. A set of one or more controllers configured for use with the display device of claim 1 , the set of one or more controllers comprising:

one or more processors; and

memory storing one or more programs for execution by the one or more processors, the one or more programs including instructions for:

placing the tunable light source in the first state for concurrently providing the first light having the first wavelength and the third light having the third wavelength; and

placing the tunable light source in the second state for concurrently providing the second light having the second wavelength and the fourth light having the fourth wavelength.

18. A method, comprising:

at a tunable light source operable in different states including a first state and a second state:

placing the tunable light source in the first state for providing (i) first light having a first wavelength and (ii) third light having a third wavelength distinct from the first wavelength;

placing the tunable light source in the second state for providing (iii) second light having a second wavelength distinct from the first wavelength and the third wavelength and (iv) fourth light having a fourth wavelength distinct from the first wavelength, the second wavelength, and the third wavelength, wherein the first wavelength and the second wavelength correspond to a first color band and the third wavelength and the fourth wavelength correspond to a second color band distinct from the first color band;

receiving, with a diffractive optical element, the first light and redirecting the first light;

receiving, with the diffractive optical element, the second light and redirecting the second light;

receiving, with the diffractive optical element, the third light and redirecting the third light;

receiving, with the diffractive optical element, the fourth light and redirecting the fourth light, wherein the diffractive optical element has (a) a first focal length for the first light (b) a second focal length, distinct from the first focal length, for the second light, (c) a third focal length for the third light, and (d) a fourth focal length, distinct from the third focal length, for the fourth light; and

receiving and modifying, with a wavelength-dependent optical element, at least one of the first light and the third light so that both the first light and the third light have a common virtual display plane.